# Relationship of the Moon to the Earth

Chapter overview

2 weeks

In Gr. 4 learners covered the basic facts about the Moon: its lack of air and water, size relative to the Earth and its position with respect to the Sun. They also observed the Moon's phases. In Gr. 6 learners learnt about the Moon's motion in space: it revolves around the Earth whilst rotating on its spin axis. In this chapter, learners will develop an understanding of how the phases of the Moon are related to the relative positions of the Earth, Moon and Sun. They will also be introduced to the concept of gravity, (covered in more detail in Gr. 9: Energy and Change strand), and the influence of the Moon's and Sun's gravitational pulls on the Earth's oceans which result in tides.

The main aims of this chapter are to ensure that learners understand the following:

• The Moon is smaller than the Earth and orbits around the Earth in 27.3 days as the Earth revolves around the Sun.
• The Moon is held in orbit around the Earth by the force of gravity. In turn the Earth and all the other planets in the solar system are held in orbit around the Sun by the force of gravity.
• All masses experience the force of gravity, and the size of the force exerted is dependent upon the mass of the objects and their distance from each other.
• The combined gravitational pull of the Moon and the Sun on the Earth's oceans cause the ocean tides.

2.1 Relative positions (1.5 hours)

 Tasks Skills Recommendation Activity: Moon revision quiz Recalling, stating Optional revision Activity: Observe the Moon Observing Suggested Activity: Total Solar Eclipse Observing, analysing Suggested

Note: There are three additional activities included only in the Teacher's Guide in this section. They are:

• Activity: Google the Moon(Replacement activity for "Observe the Moon" if computers and internet are available.)
• Activity: Hands-on Moon Phases (Optional extension activity, revision of Gr. 6 material.)
• Activity: Month Long Moon Observation (Optional extension activity. This is a repeat of an activity done in Gr. 6. This reminds learners of the Moon's phases but does not yet link them to the relative positions of the Sun/Earth/Moon.)

2.2 Gravity (2 hours)

 Tasks Skills Recommendation Activity: Demonstrating the Moon's orbit around the Earth Investigating, observing CAPS suggested Activity: How heavy would you be on other planets? Calculating, measuring Suggested

Note: There is an additional investigation included only in the Teacher's Guide in this section. It is:

• Investigation: Dropping objects (Optional extension activity)

2.3 Tides (2.5 hours)

 Tasks Skills Recommendation Activity: Reading a tide chart Reading graphs Suggested Activity: Dance of the tides Working in groups, investigating, analysing Suggested Activity: Spring and neap tides Observing, analysing CAPS suggested Activity: The effect of tides on shoreline ecosystems Researching, analysing, writing CAPS suggested Activity: How good a fisherman are you? Analysing data Suggested

Note: There are two additional activities included only in the Teacher's Guide in this section. They are:

• Activity: Tides poster (Optional, fun activity)
• Activity: Make a tide wheel (Optional activity)

• How long does it take for the Moon to orbit the Earth?
• What keeps the Moon in orbit around the Earth?
• What causes tides on Earth?

Earth's only natural satellite is simply called the Moon because people didn't know other moons existed until Galileo Galilei discovered four moons orbiting Jupiter in 1610. Other moons in our solar system are given names so they won't be confused with each other.

The Moon is the most obvious feature in our night sky and has captivated people for thousands of years. Ancient cultures recorded the apparent motion of the Moon through the sky and made calendars which used the phases of the Moon to mark the months. In fact some religious calendars still use a lunar (Moon) based calendar rather than the official solar (Sun) based calendar used today in South Africa and most of the Western world (called the Gregorian calendar). The Moon's influence on the Earth is also important to us in other ways as you will discover in this chapter.

A spacecraft, called Ladee, was launched in 2013 to orbit the Moon to gather information about the lunar environment. Have a look at this infographic detailing the mission. http://www.nasa.gov/mission_pages/ladee/infographic/#.Ui4uOCLtnb6

Watch the original footage of Apollo 11 landing on the Moon in 1969.

## Relative positions

• moon
• lunar
• eclipse

You learnt about the Moon in Grades 4 and 6. Lets see what you can remember!

# Moon revision quiz

This is an activity to review material covered in Grades 4 and 6. It is a short, optional activity.

INSTRUCTIONS:

Fill in the gaps in the Earth-Moon comparison table below using the word bank.

 Word bank: rock, soil and water rock and lunar soil reflects absorbs Sun Earth an no larger smaller 24 27.3
 The Earth The Moon Surface consists of _____. Surface consists of _____. Is ____ _than the Moon Is _____ than the Earth Is visible because it _____ light from the Sun hitting it. Is visible because it _____ light from the Sun hitting it. Is in orbit around the _____. Is in orbit around the_____. Spins on its axis once ever y _____ hours. Spins on its axis once every _____ days Has _____ atmosphere. Has _____ atmosphere.

 The Earth The Moon Surface consists of rock, soil and water. Surface is consists of rock and lunar soil. Is larger than the Moon Is smaller than the Earth Is visible because it reflects light from the Sun hitting it. Is visible because it reflects light from the Sun hitting it. Is in orbit around the Sun. Is in orbit around the Earth. Spins on its axis once every 24 hours. Spins on its axis once every 27.3 days. Has an atmosphere. Has no atmosphere.

The Moon is actually covered in a layer of lunar 'soil' called regolith. This is why you can see astronauts' footprints on the Moon. Lunar 'soil' has different properties to soil on Earth, most significant is that terrestrial soil has organic matter in it.

Let's now take a closer look at the surface of the Moon.

# Observe the Moon!

In this activity learners look in detail at the surface features of the Moon. The photographs show images of both the near side and far side of the Moon which look quite different and learners should be encouraged to compare the two.

INSTRUCTIONS:

1. Study the images of the Moon.

QUESTIONS:

Does the Moon's surface have any oceans or lakes?

No, the surface is all solid rock and lunar soil (regolith).

What do you notice covering much of the Moon's surface?

Craters.

Some areas look dark and others look lighter, the dark areas are called maria (singular mare) meaning seas, as astronomers initially thought that these areas were seas on the surface. The bright areas are called highlands as they are higher than the maria. On what side of the Moon (near or far) are there more dark areas (maria)?

The near side has more maria.

Humanity got its first view of the far side of the Moon in 1959 when the Soviet Union launched the small spacecraftLuna 3. This was the first probe to get to the far side of the Moon and photograph it.

This is an optional activity to do with your learners if you have internet access. You can use it as an alternative to the previous activity where the images are provided.

This activity is a nice way to incorporate ICT into a science lesson. The Google Earth software can be downloaded for free directly onto your computer from the following web address: http://www.google.com/earth/explore/products/ The software contains an interactive map of the Earth, Moon and Mars. Switching to 'Moon mode', learners can look at the surface of the Moon in amazing detail, rotating, and zooming in or out of the map. Additional information about the 6 Apollo landing sites is also available with photos and short videos. Both low and high resolution images of the Moon can be studied along with contour maps to explore the heights and depths of the craters. If you do not have access to a computer then an alternative activity is to show students a variety of photographs of the Moon.

MATERIALS:

• Computer

INSTRUCTIONS:

2. Switch to Moon mode.
3. Study the images of the Moon, rotating, zooming in and out to look at the surface features.
4. Read the information about the Apollo missions and other missions to the Moon.

QUESTIONS:

1. Does the Moon's surface have any oceans or lakes?

No, the surface is all solid rock.

1. What do you notice covering much of the Moon's surface?

Craters.

1. Some areas look dark and others look lighter, the dark areas are called maria (singular mare) meaning seas, as astronomers initially thought that these areas were seas on the surface. The bright areas are called highlands as they are higher than the maria. Rotate the image of the Moon around. On what side of the Moon (near or far) are there more dark areas (maria)?

The near side has more maria.

1. In what mare or sea did the Apollo 11 mission land?

Sea of Tranquility. (Latin: Mare Tranquillitatis).

The Earth, just like all the other planets in the solar system, travels around the Sun, completing one revolution every year. As the Earth travels around the Sun it has a companion in space: our Moon!

The time that it takes an object to make one complete orbit around another object, relative to the stars, is called the orbital period or synodic period.

The Moon orbits around the Earth completing one revolution every 27.3 days. Our Moon rotates on its own axis and experiences daytime and dark nighttime just like the Earth does. However, the Moon spins much more slowly than the Earth does and completes one rotation on its axis once every 27.3 days. Did you notice that the Moon takes the same amount of time to spin on its axis as it does to orbit completely around Earth? This means that from the Earth, we always see the same side of the Moon (called the 'nearside'). The side we do not see from Earth, called the 'farside', has been mapped during space missions to the Moon.

The sidereal period of the Moon is mentioned in the previous paragraph, namely the Moon completes an orbit around the Earth about once every 27.3 days. This is not to be confused with the Moon's synodic period, which is 29.5 days. The synodic period of the Moon is due to the Earth moving in its orbit about the Sun at the same time, and so it takes slightly longer for the Moon to show the same phase to Earth.

Why do we only see one side of the Moon? (video)

The Moon revolves around the Earth at an average orbital speed of about 1 km per second!

Viewed from above, the Moon moves in an anti-clockwise direction around the Earth. The Moon's orbit is not a perfect circle, it is elliptical, so its distance from Earth varies as it revolves around the Earth. The average distance is about 385 000 km, which is about 60 times the radius of the Earth itself. For comparison, the Earth's average distance from the Sun is 149 597 871 km, or about 23 481 times the radius of the Earth. You can see now why the Moon is called Earth's close companion!

In Gr. 6, learners would have done some activities to act out the revolution of the Moon around the Earth as the Earth orbits the Moon. You can view this content online at www.thunderboltkids.co.za. Here is a link to the actual content: http://bit.ly/15wPXbE. Here is an image of the activity if you would like to repeat it with your learners to reinforce the relative positions of the Earth, Moon and Sun.

In the diagram, the Sun and Sun-Earth distance are not drawn to scale, the Sun would be MUCH larger than in this image and the distance between the Sun and Earth would also be MUCH larger.

The Sun is on the right.

The following table summarises some useful information about the Sun, Earth and Moon.

 Characteristic Sun Earth Moon Relative position Is at the centre of our solar system Orbits the Sun once every 365.25 days Orbits the Earth once every 27.3 days Rotation Spins on its own axis roughly once every 28 days Spins on its own axis once every 24 hours Spins on its own axis once every 27.3 days Distance from orbited body - 23 481 Earth radii from the Sun 60 Earth radii from Earth Size Diameter is roughly 100 times the Earth's diameter - Diameter is roughly ⅓ times the Earth's diameter

Did the Earth have two Moons?

We have now looked at the relative positions and movement of the Earth, Moon and Sun. Let's extend this knowledge to learn about a solar eclipse.

# Total Solar Eclipse

In this activity, the idea that the apparent size of an object depends upon its distance from an observer is reinforced. Learners will find that although the Sun is much much larger than the Moon, it appears about the same size in the sky because it is much more distant than the Moon.

INSTRUCTIONS:

Look at the image below. It shows a total solar eclipse which you learnt about in Gr. 6. This happens when the Moon passes directly in front of the Sun and blocks the Sun's light. The bright light from the Sun is blocked, allowing us to see the very faint outer edge of the Sun's atmosphere, called the corona. We normally cannot see the corona as it is swamped by the bright light from the Sun. When you look at the size of the Moon in the sky compared with the size of the Sun in the sky you see that they are very similar. We call this the angular size. This is because the Moon is much closer than the Sun. The Moon appears large enough from Earth to totally block out the Sun's light.

What is a solar eclipse? (video)

QUESTIONS:

Which in reality is larger, the Moon or the Sun?

The Sun is larger.

Which is further away, the Moon or the Sun?

The Sun is further away.

How do the angular sizes of the Moon and the Sun compare when viewed from the Earth's surface?

They are almost the same.

Why is this the case?

Although the Sun is much larger than the Moon, it is much more distant. As objects appear to look smaller and smaller the further away they are, the Sun appears smaller than it is in reality. The Moon also appears smaller than it is in reality, however it is much closer to the Earth than the Sun is and so its apparent size isn't reduced as much as the Sun's is. Just by chance, the Sun and moon are currently at distances where they have the same angular size viewed from the Earth's surface.

A total solar eclipse occurs when the Earth, Moon and Sun are aligned in a straight line with the Moon placed in between the Earth and the Sun. Just by chance, the Sun and Moon are currently at distances where they have the same angular size viewed from the Earth's surface. If the angular size of the Moon were smaller, it would not be large enough to completely block the Sun and we wouldn't have total solar eclipses! The picture below shows the relative alignment of the Sun, Earth and Moon during a solar eclipse.

Why does the Moon sometimes appear red?

We can also get a lunar eclipse. This is when the Sun, Earth and Moon line up with the Earth in the middle.

Read more about why the Moon can appear red during a lunar eclipse. http://earthsky.org/space/why-does-the-moon-look-red-during-a-total-lunar-eclipse

See how a lunar eclipse compares to a solar eclipse in the diagram. In this case, the Earth blocks the sunlight from reaching the Moon's surface, making the Moon appear dark in the night sky.

Find out when the next lunar eclipses will take place. http://eclipse.gsfc.nasa.gov/lunar.html

The Moon is slowly moving away from the Earth at a rate of 3.8 cm per year (the Moon's orbit is getting larger). In about 563 million years time its angular size on the sky will have decreased so much that it will no longer be large enough to produce total solar eclipses!

Learners will most likely be familiar with the change in the Moon's appearance over the course of a month, the lunar phases. Each lunar phase cycle from New Moon to New Moon takes 29.5 days, which is slightly longer than the Moon takes to complete one revolution around the Earth (27.3 days). This is because during the 27.3 days it takes for the Moon to revolve around the Earth, the Earth is moving along in its orbit. In order for the Moon to appear at the same phase as viewed byan observer on Earthit needs to travel slightly further than 360 degrees around the Earth and in order for it to be aligned such that there is a New Moon it takes about an extra 2 days.

http://www.sumanasinc.com/webcontent/animations/content/sidereal.html shows a nice animation demonstrating the difference between the orbital period of the Moon which defines the sidereal month (27.3 days) and the lunar phase cycle which defines the synodic month (29.5 days).

Observers on Earth see the same phase where ever they are positioned on Earth. However, the phases (apart from New Moon and Full Moon) look different to an observer in the Northern and Southern hemispheres. We in the Southern Hemisphere view the Moon "upside down". This is important to note because $$\text{99}$$% of all textbooks and online references include the Northern Hemisphere view of the phases and this is not what learners will see when they view the Moon for themselves. An awareness of this is crucial to reduce learner confusion when looking online and in generic textbooks.

The activity below is an optional extension activity which links the relative positions of the Earth, Sun and Moon to the phases of the Moon observed.

Activity: H ands-on Moon Phases

As the Moon revolves around the Earth, the side facing the Sun is always illuminated, just as Earth's daylight side is illuminated by the Sun. However, from the Earth's surface we do not see a half Moon lit up all the time. Instead, we see a change in the amount of the Moon which is lit up by the Sun.

In this activity learners will learn that the relative position of the Earth, Moon and Sun determine what phase of the Moon is observed. Learners will use a lamp to represent the Sun. The learners will represent the Earth and a styrofoam ball stuck on a pencil will represent the Moon. If you cannot get hold of a styrofoam ball then you can use an orange instead. They will vary the location of the Moon in its orbit around the Earth and observe the phase of the Moon. A dark room is needed for this activity. If necessary darken the classroom with bin bags or curtains. In the centre of the room place an unshaded lamp to represent the Sun.

Ideally learners should work in pairs for this activity, so that one learner can draw their observations while the other learner holds the ball in place. Ensure that learners hold the balls slightly above their heads so that they do not cast shadows over the ball. In this exercise we make the assumption that the Earth remains in the same spot while the Moon orbits around the Earth.

Before starting the activity, explain to learners the names of the Moon phases and draw them on the blackboard: New Moon (entirely dark), Full Moon (entirely lit), Crescent Moon (mostly dark), Gibbous Moon (mostly lit) and First Quarter Moon (left half lit) and Third Quarter Moon (right half lit). [Note that the first and third quarter appearances listed here are for the Southern Hemisphere only]

MATERIALS:

• pencil (2 per pair)
• one lamp that can shine in all directions (i.e., a lamp base with a bare 100 to 150 Watt bulb and no lampshade)
• styrofoam balls (1 per pair)
• black plastic bags (and tape) or curtains to darken the classroom
• sheet of paper (1 per pair)

INSTRUCTIONS:

1. You will work in pairs for this activity.
2. Place a lamp representing the Sun in the centre of the classroom. Even for a large classroom, you should only use one bright lamp placed in the middle of the classroom otherwise you will have shadowing effects that may ruin the results.
3. Darken the room if needed by taping dark plastic bags to the windows or closing the curtains.
4. Stick one of the pencils into your styrofoam ball so that you can hold the ball up by the pencil end. This ball represents the Moon.
5. All learners must stand in a circle around the central light, with your partner next to you in the circle.
6. Directly face the light in front of you. One of you should hold the ball at arms length, slightly above your head, and the other should hold the pencil and paper. The person holding the ball represents the Earth.
7. If you are holding the ball, move the ball from left to right (keeping still) and observe how much of it is lit by the light as you move it around. Now let your partner do the same.
8. Look at the different phases of the Moon drawn on the blackboard by your teacher (New Moon, Full Moon, First quarter, Third quarter).
9. One of you should now hold the ball and position it until from your point of view it looks completely in shadow. This represents New Moon.
10. The member of the pair not holding the ball should now draw the relative positions of the Sun, Moon and Earth, and write down the Moon phase corresponding to these positions.
11. Swap the person holding the ball and the pen/paper, and position the ball such that it is fully lit and looks like a Full Moon. The member of the pair not holding the ball should now write down the relative positions of the Sun, Moon and Earth.
12. Repeat this for all the phases listed in 8.
13. Look at the crescent Moon on the blackboard. Find out what positions you can place the ball in to see it lit up like a crescent (less than half lit).
14. Swap positions again and this time find out what positions you can place the ball in to see it lit up in a gibbous phase (more than half lit).

QUESTIONS:

1. In what position do you need to place the Moon in order to see a New Moon?

The Moon needs to be placed directly in between the Sun and the Earth.

1. In what position do you need to place the Moon in order to see a full Moon?

The ball needs to be place d directly opposite from the Sun, with the Earth in between the two.

1. In what positions can you place the Moon in order to see a crescent?

Any position as far as 90 degrees either side of the N ew Moon position (to the left or right).

Another optional activity that learners may complete in their own time is a month long Moon observation. They may have already completed this activity in Gr. 6 but it is included here in case you wish to conduct a Moon observation as an additional activity.

Activity: Month long Moon observation

This activity can be conducted by students at home.The activity takes a month (30 days) to complete, therefore it can be done while other material is covered in class. Students may need reminding to complete their observations every day. Start the observations at New Moon so that the learners can follow the phases of the Moon in order. The dates of New Moon can be obtained online at http://aa.usno.navy.mil/data/docs/MoonPhase.php Once learners have completed all their observations you can discuss in class what they saw and ask them questions as to why they think the Moon changes its appearance.

MATERIALS:

• Moon observation chart
• pencil

INSTRUCTIONS:

1. Draw up an observation chart with blank circles to represent the moon each day.
2. Go out and observe the Moon. (You may have to do this during the day or during the night depending on when the Moon is visible).
3. Always stand in approximately the same spot and face the same direction (either south or north). Look from east to west and find the Moon.
4. Draw how the Moon looks by shading in the circles to reflect the shape of the Moon in your observation chart.
5. Note the date and time of your observation.

Help: For example, if you can see the whole Moon, you do not need to shade in any part of the circle. If you can only see half of the Moon, shade the side of the Moon that you cannot see in the circle for that day. If you cannot see the Moon at all on a day, indicate this on your journal and also write down why you could not see the Moon.

## Gravity

Before introducing the concept of gravity ensure that learners fully understand what is meant by a force (a push or a pull). Run through some everyday examples of forces that learners encounter such as pushing a trolley at the supermarket, or pushing or pulling their friends!

Strictly speaking when talking about "gravity" we are specifically referring to the gravitational force of attraction that occurs between the Earth (or another celestial body like a planet) and other objects, as opposed to the gravitational force in general which acts between any two objects with mass. For example, we would refer to the gravitational force acting to attract objects to the Moon as the Moon's gravity, but we would not generally refer to the gravitational force acting to attract things to ourselves as 'our gravity'.

• gravity
• mass
• weight
• acceleration due to gravity
• gravitational force

The word gravity is used to describe the gravitational pull (force) an object experiences on or near the surface of a planet or moon. The gravitational force is a force that attracts objects with mass towards each other. Any object with mass exerts a gravitational force on any other object with mass. So, the Earth exerts a gravitational pull on you, the desks in your classroom and the chairs in your classroom, holding you on the surface and stopping you from drifting off into space.

The Earth's gravity pulls everything down towards the centre of the Earth and so when you drop an object like a book or an apple it falls to the ground. However, do you know that you, your desk, your chair, and the falling apple and book exert an equal but opposite pull on the Earth? Why do you think that these pulls don't cause the Earth to move noticeably?

The Earth has a much larger mass than a person or a desk and so it is accelerated by a much smaller amount even though the force exerted on the Earth by a desk is the same size as the force exerted on the desk by the Earth (just in opposite directions). This is why the Earth does not movecnoticeably.

Interact with this simulation to see the relationship between gravity and the masses of the object and distance between them. http://phet.colorado.edu/sims/html/gravity-force-lab/latest/gravity-force-lab_en.html

The PhET simulation in the visit box can be used to very easily demonstrate how the gravitational force between two objects increases with mass and decreases as the distance between the objects increases. You can turn off the values, and use the position of the little figures tugging on the ropes to qualitatively demonstrate the relationships.

The gravitational force between two objects decreases as the objects move further apart. If you double the distance between two objects the gravitational force between them decreases by a factor of four. Similarly if you triple the distance between them, the gravitational force between them decreases by a factor of nine. This explains why we are stuck to the Earth rather than the Sun. The Sun is 333 000 times more massive than the Earth and its gravity is much stronger than the Earth's. However, we are so far away from the Sun that the gravitational force the Sun exerts on us, is much smaller than the gravitational force the Earth exerts on us.

Watch Felix Baumgartner's supersonic freefall back to Earth.

The Moon is held in orbit around the Earth by the gravitational force between the Earth and the Moon. Similarly, the Sun's gravity holds the Earth in orbit around the Sun. Lets do an activity to demonstrate the Moon's orbit around the Earth.

# Demonstrating the Moon's orbit around the Earth

In this activity learners will demonstrate the orbit of the Moon around the Earth using a ball tied to a rope swung around their heads. They will demonstrate what would happen to the Moon if there were no gravity by letting go of the rope.

Safety tip: Do this activity outside or in the school hall if possible so that learners can spread out. This will help them avoid hitting each other when the balls are released. If this is not possible take it in turns to do this demonstration or have only a few learners do this demonstration so that no one is hit by a flying ball!

--> MATERIALS

• rope
• ball (tennis balls are ideal)

INSTRUCTIONS

1. Tie a ball to the end of a piece of rope. You may have to wrap the rope around the ball a few times to do this.
2. Hold the rope up high above your head and swing the rope around in a horizontal circle.
3. Let go of the rope and observe what happens.

QUESTIONS:

How can you describe the movement of the ball as you swing it around?

The ball moves around in a complete circle.

The rope pulls the ball inwards towards the centre of the circle keeping the ball moving in a circle. What force holds the Moon in orbit around the Earth?

The gravitational attraction between the Earth and the Moon.

What happens to be ball when you let the rope go? \

If the rope is released the ball flies off in the direction it was travelling in just as the rope was released.

What does this represent in terms of the Earth and the Moon?

This represents that the gravity keeps the Moon in it's path around the Earth. Without it, the Moon would move away from its path.

All the components in our Universe are held together by gravity. In summary we can say:

• The greater the mass of the objects, the stronger the gravitational pull between them.
• The closer objects are, the stronger the gravitational pull between them.

Move the Sun, Earth, Moon and space station to see how it affects their gravitational forces and orbital paths. http://phet.colorado.edu/en/simulation/gravity-and-orbits

### Weight

The content here on weight is not specified for Gr. 7 level in CAPS, and only appears in Gr. 9 in CAPS. However, as learners confuse mass and weight very easily, this has been included as enrichment material at this level. You can decide whether you want to cover this content with your learners or not. It is not to be assessed in Gr. 7.

The weight of an object is the force acting on it due to gravity. Weight is not the same as mass although the two words are often confused in everyday language.

The mass of an object is the amount of matter in the object, it tells you how many particles you have. Do you remember that we briefly spoke about atoms in Matter and Materials? So, for example, the mass of a wooden block tells us how many atoms there are. Mass is measured in kilograms (kg) and is independent of where you measure it. A wooden block with a mass of 10 kg on Earth also has a mass of 10 kg on the Moon.

However, an object's weight can change as it depends on the mass of the object and also the strength of gravity acting on it. Weight is measured in Newtons (N). For example the Earth exerts a gravitational force of about 10 Newtons for every kilogram of mass on its surface. So, a person with a mass of 50 kg has a weight of 500 N on the surface of the Earth.

The Moon also has its own gravity. The strength of gravity on the surface of the Moon is one-sixth that of the Earth, and so you would weigh one-sixth of what you do on Earth on the Moon. On Jupiter you would weigh 2.5 times more than you do on Earth as Jupiter's gravity is 2.5 times that of the Earth's. Even though you would weigh different amounts (and feel lighter on the Moon and heavier on Jupiter) your actual mass would stay the same in both cases.

Answer the following questions to check your understanding of mass and weight:

Lindiwe has a mass of 50 kg on Earth. What is her mass on the Moon?

50 kg as the mass of an object is independent of position.

Andrew has a mass of 60 kg on Earth, what is his weight in Newtons on Earth?

600 N (60 x 10)

How much would Andrew weigh on the Moon?

100 N (60 x 10/6)

Would Lindiwe feel heavier or lighter on the Moon?

She would feel lighter on the Moon, even though her mass is the same on the Moon.

# How much would you weigh on other planets?

In this activity, learners calculate what their weight would be on the seven other planets in our solar system. Although their mass remains the same, they will "feel" lighter or heavier because of the differences in the gravitational field strength on the surfaces of the other planets. You should emphasise that their mass always remains the same, but only their weight varies. If you do not have access to weighing scales you can either ask learners to estimate their mass or provide them with an example number.

MATERIALS:

• weighing scales
• calculator

INSTRUCTIONS:

1. Measure your mass in kilograms using weighing scales. Record the value in the table below.
2. Look at the table below, it shows how strong the gravity is on each of the planets in our solar system.
3. Calculate your weight on each of the planets and enter it into the table below.

Hint: On Earth each kilogram weighs 10 Newtons. So if your mass is 50 kg then you weigh 50 x 10 = 500 N on Earth. If the strength of gravity on a planet is half the strength of the Earth's gravity then you would weigh half of what you weigh on Earth on that planet.

 Planet Your mass (kilograms) Strength of gravity relative to Earth Your w eight (Newtons) Earth 1 Mercury 0.378 Venus 0.907 Mars 0.377 Jupiter 2.36 Saturn 0.916 Uranus 0.889 Neptune 1.12

Example answers for a 50kg learner

 Planet Your mass (kilograms) Strength of Gravity relative to Earth Your weight (Newtons) Earth 50 1 500 Mercury 50 0.378 189 Venus 50 0.907 453.5 Mars 50 0.377 188.5 Jupiter 50 2.36 1180 Saturn 50 0.916 458 Uranus 50 0.889 444.5 Neptune 50 1.12 560

QUESTIONS:

On which planets would you feel heavier than you do on Earth?

You would feel heavier on Jupiter and Neptune.

On which planets would you feel lighter than you do on Earth?

You would feel lighter on Mercury, Venus, Mars, Saturn and Uranus.

A note on falling objects

A useful way to demonstrate the Earth's gravity is to look at falling objects. An optional extension activity is included below in which learners drop a variety of objects. You can take a vote from the class to see whether learners think that an apple or bag of sugar would hit the ground first. (Answer: they would hit the ground at the same time as long as air resistance is negligible.) It is very likely that learners will have the preconception that heavier items fall faster. It is not important at the moment that the learners answers are correct and do not try to lead them to the correct answer. They will hopefully discover it for themselves in the following experiment.

Investigation: Dropping objects

In this investigation learners need to work in pairs. They will initially drop a whole apple and half an apple from the same height at the same time. They will then further experiment with balls of different masses (but the same size) and balls of the same mass (but different volumes). It is very hard to drop objects at exactly the same time so that they hit the floor simultaneously so let the learners repeat the experiment several times until they are confident that they are dropping the objects at the same time. If it is hard for them to see which object hits the ground first, suggest to that learners they listen for the number of sounds they hear - one or two - when the objects hit. Learners might need to repeat this investigation many times since it most likely contradicts their preconceptions! Safety tip: It is probably a good idea to have the apples cut in half ahead of time.

Once the learners have finished their experiment you can demonstrate the effects of air resistance by dropping a hammer and a feather. Have the learners take a vote on what will happen when you drop the hammer and feather. Be ready to explain to learners that air resistance slows the fall of the feather and that if there were no air resistance the two would fall at the same rate and hit the floor at the same time.

INVESTIGATIVE QUESTION: Do different objects fall at the same rate?

HYPOTHESIS:

What do you think will happen?

IDENTIFY VARIABLES:

What are you keeping constant in this experiment?

The height at which objects are dropped.

What are you changing in this experiment?

The type objects that are being dropped, in particular the mass and volume of the objects.

MATERIALS AND APPARATUS:

• hammer
• feather
• apples (one and a half per pair)
• knife (if needed to cut the apples in half)
• two balls of the same mass, different volumes (one set per pair)
• two balls of the same volume, different masses (one set per pair)

METHOD:

1. Work in pairs, take it in turns to be the person who drops an object (experimenter) and the person who observes the object dropping (observer).
2. Fill in the "prediction" column in the table below.
3. Experimenter: stand on top of a chair or desk and take an apple in one hand and apple half in the other hand.
4. Experimenter: hold the two up at the same height in front of you and drop them at exactly the same time.
5. Observer: note what happens, in particular which lands first.
6. Swap positions and repeat the experiment using two balls which have the same mass but different volumes.
7. Swap positions and repeat the experiment using two balls which have the same volume but different masses.
8. Your teacher will now do a demonstration for you and drop a hammer and a feather. Before your teacher drops the hammer and feather, fill in the prediction column for the hammer and feather drop.
9. Write down what happened with the hammer and feather and answer the questions below.

RESULTS AND OBSERVATIONS

In the table below, fill in what you think will happen in the "prediction" column before you conduct your experiment. Assuming that you drop each pair of objects from the same height at the same time, what do you think will happen? Which do you think will land first?

 Objects Prediction Observation Apple and half apple Balls: same mass, different volume Balls: different mass, same volume Hammer and feather

EVALUATION:

Learner dependent answer. Example answers could include: It is difficult to drop objects at exactly the same time. It would be better to drop the objects from a greater height. Air resistance could have affected the results and it would be better to drop the objects in a vacuum.

CONCLUSIONS:

Learners should have found that the apple and half apple hit the floor at the same time. They should also have found that the balls of the same mass hit the floor at the same time and also the balls of the same volume hit the floor at the same time. From this they should conclude that all objects dropped, fall at the same rate no matter what their shape or size if air resistance can be ignored. (Advanced: they accelerate at the same rate). In the case of the hammer and feather drop, learners should have found that the hammer landed first. This is because of the effects of air resistance slowing the feather's fall.

QUESTIONS:

1. Which landed first, the apple or the half apple?

They should have both landed at the same (or close to the same) time.

1. Considering the balls of the same mass, which landed first, the larger one or the smaller one?

They should have both landed at the same time.

1. Considering the balls of the same volume, which landed first, the heavier one or the lighter one?

They should have both landed at the same time.

1. Why do you think the two balls dropped always landed at the same time?

In an ideal situation, all objects dropped from the same height will land at the same time, this is because the Earth's gravity causes the same acceleration for everything no matter how heavy it is or its volume.

Advanced Teacher Note: According to the universal law of gravitation, the Earth's gravitational force pulls down on an object with a force that is proportional to the (gravitational) mass of the object and the (gravitational) mass of the Earth. In all cases the mass of the Earth is the same and so any differences in the gravitational force depends only upon the difference in the gravitational mass of the objects being dropped.

According to Newton's second law, the force exerted on an object, F, is given by F=ma where m is the inertial mass of the object and a is the acceleration produced by the force F. All objects resist movement when acted on by a force. This resistance is called inertia and arises because an object has (inertial) mass.

When dropped, a heavier object experiences a greater gravitational force as it has a greater (gravitational) mass, but also resists harder as well as its inertial mass is larger. Lighter objects experience a smaller gravitational force and a smaller inertia.

We have, Force=mia = mgg where g is independent of the falling object, where mi= inertial mass and mg = gravitational mass.

So the resulting acceleration an object experiences is given by a = (mg/mi) x g

As the value of g is independent of the falling object, the acceleration is given by the ratio of the gravitational and inertial masses. It turns out that the measured acceleration of all objects in the Earth's gravitational field is the same, they all fall at the same rate. This implies that the ratio between the gravitational mass and inertial mass is the same for all objects. By setting the value of G, the gravitational constant appropriately we can set the two masses equal to each other, a rather remarkable result since the gravitational mass is what causes the object to accelerate and the inertial mass opposes the acceleration!

1. Why do you think that the hammer landed before the feather?

In a real situation, the air around us affects how objects fall. As an object moves through the air, it must push the air out of the way, it experiences air resistance. The feather is much lighter than the hammer and so the effect of air resistance is much larger for the feather. The net force acting downwards on a falling object is the force due to gravity - force due to air resistance. As the feather is much lighter than the hammer, the net force acting on it will be less and so it will experience a smaller acceleration towards the ground and fall more slowly.

Advanced Teacher Note: Air resistance is a drag force acting to slow the object down. The size of the force depends upon the velocity of the falling object squared, the surface area of the falling object and the density of the fluid it is falling in (in this case air). Very light objects are slowed by air resistance, like feathers or thin sheets of paper. This is because their weight is very small compared with the air resistance. Very large objects are also slowed by air resistance. This explains why a parachute slows your fall. Before you open a parachute only a small amount of air needs to be pushed out of the way as you fall. After opening, the wide parachute must push much more air out of the way and the air resistance increases, slowing you down.

A note on Weightlessness

The term weightless causes a lot of confusion for learners. The confusion of a person's actual weight with one's feeling of weight is the source of many misconceptions. Weightlessness refers only to someone's sensation of their weight, or lack thereof. Weightlessness is a feeling experienced by someone when there are no external objects touching the person exerting a push or pull upon them, (we call these contact forces because they arise due to things being in contact or touching each other).

The weight of a person is the force of gravitational attraction to the Earth, which that person experiences. Someone in free fall, feels weightless but they have not lost their weight, they are still experiencing the Earth's gravitational attraction.

Learners are also often confused as to why astronauts in orbit around the Earth float in their spacecraft. One common misconception is that there is no gravity in space and so the astronauts can float. In actual fact, in low Earth orbit the Earth's gravity is about $$\text{90}$$% of its strength at the surface of Earth. The only reason the astronauts float is because they are in free fall and their spacecraft is also in free fall with them, falling at the same rate. Therefore, the astronauts appear to float when compared with the spacecraft because they are both falling at the same rate. Another example is how orbiting spacecraft are essentially in free fall as there is 'nothing' retarding their motion towards to centre of the Earth, but because of their orbital velocity, they never actually move closer to the Earth.

A useful link to a video of someone experiencing free fall is given below:

Watch Felix Baumbartner's skydive. He experienced free-fall or weightlessness.

### The Moon's gravity

As you have already discovered, the Moon, like any other planet or moon, has its own gravity. The strength of gravity on the surface of the Moon is one-sixth that of the Earth, and so on the Moon you would weigh one-sixth of what you do on Earth. Due to the weak gravity on the Moon, you would be able to jump six times higher than usual! The astronauts had to learn to walk in strange ways (such as leaping or hopping) to move about on the surface of the Moon.

Neil Armstrong walking on the Moon (video)

As we will find out in the next section, the Moon's gravity not only affects humans walking on the Moon, but also influences the Earth.

The Moon's gravity affects humans on Earth. The tug of the Moon's gravity decreases a person's weight by the equivalent of a few grams on the surface of the Earth!

## Tides

Learners are often confused about the differences between waves and tides. Waves on the surface of the oceans, seas or lakes are caused by the wind and are independent of tides. Tides cause the overall water level to change with time. At high tide the sea is really high up on the beach, at low tide it is really far out. The water level gradually changes between the two and the cycle repeats daily with two high tides and two low tides at a given point in 24 hours. A tidal wave, or tsunami, is caused by a sudden disturbance, such as an earthquake and is unrelated to tides.

• tides
• tidal bulge
• spring tides
• neap tides

Tides are the predictable, repeated rise and fall of sea levels on Earth. If you look closely you will notice that the height of the surf at any beach varies slowly with time. When the sea is far out and there is lots of sand exposed, it is called low tide. You can see an example of low tide in the photo.

Following low tide, the water gradually comes further up on the beach until it reaches its highest level, this is called high tide. After high tide the water level gradually drops again until it goes back to low tide. This pattern repeats over and over again. You can see an example of low and high tide at the same beach in the pictures below.

In general there are are two low and two high tides per day on the sea, which can be observed on the beaches or even in estuaries. The times of high and low tides are not exactly the same every day, they occur roughly one hour later each day.

Tides can be predicted and low and high tide times are published in tide tables. Fishermen use this information to plan when they will fish. Surfers also use this information so they can plan the best times to go surfing as each beach has a particular time when the sea level is just right for producing excellent surfing waves.

Some lakes and rivers also have tides!

This activity provides a chart showing tidal data for one week in Cape Town. This activity gives learners the opportunity to read and interpret data from the chart.

INSTRUCTIONS:

1. Look at the chart above, it shows the predicted times of low and high tide for one week in Cape Town.
2. The peaks represent times of high tide and the heights are listed in metres along with the time of high tide. The troughs represent the times of low tide.

QUESTIONS:

How many peaks appear per day in the chart?

Two

What do these correspond to? High or low tide?

High tide

How many troughs appear per day in the chart?

Two

What do these correspond to? High or low tide?

Low tide.

What is the height in metres of the highest low tide during the week?

0.7 m

When does the lowest high tide occur? (date and time)

Friday 3rd May, 10.17 am.

What height is the lowest high tide?

1.35 m

The following photo is of a small harbour in Cape Town with a boat moored. These photos were taken on Monday 29 April.

1. What time of day, was the photo taken of low tide?

2. What time of day was the photo taken of high tide?

1. At 11:47am.

2. At 6:05pm

Timelapse of a shore from low to high tide.

So you now know that all seas have tides, why do you think this is? Lets do an activity to find out.

Ask learners to give you their answer as to what causes the tides. You can write down all the answers. At this point it does not matter if the learners do not know what causes the tides, they will find out in the "Dance with the tides" activity. Once they have completed the activity, ask the learners again to see if they have changed their minds. At this point they should be aware that the gravitational pulls of the Moon and the Sun cause the tides.

# Dance of the tides

This activity requires learners to work in groups of six. One learner will represent the Earth, four learners will represent the Earth's oceans and one learner will represent the Moon. You could ask learners to wear coloured T-shirts (green for the Earth, blue for the oceans, grey for the Moon) or to pin drawings or photos of the object they are representing to their school shirts with safety pins to make it clear what they are representing.

In this activity learners will model how the Moon's gravitational pull causes tides on Earth. Be sure to explain to them that the scale of the model is not correct, for example the Moon and Earth sizes are not correct in relation to each other. You can ask learners how they think the Moon can influence the Earth. Explain that moons and planets can influence each other's spins and tilts from a distance via their gravitational pull. All objects that have mass have their own gravity, but only large objects, like planets, have enough gravity to influence each other from very large distances. Explain that you are going to model what effect the Moon's gravitational pull has on the Earth. Remember that the gravitational force exerted by an object diminishes as you go further from it. Therefore, objects that are closer to the Moon experience a greater gravitational pull towards the Moon than objects that are further away.

MATERIALS:

• Four (ideally blue) scarves or strips of fabric per group, each one needs to be about a metre in length.

INSTRUCTIONS:

Work in groups of six, one learner represents the Earth, four learners represent the Earth's oceans and one learner represents the Moon.

The learner representing the Earth: stand in an open space.

The four learners representing the oceans: take one scarf each and stand in a circle around the learner representing Earth. (One behind, one in front and one on either side).

Learner representing the Moon: stand outside the circle of "ocean" about 5 steps away from the "Earth" directly in front of one of the learners representing the ocean.

All learners apart from the Moon: turn to face the "Moon". You are now going to be "pulled" towards the Moon by the Moon's gravitational attraction! Remember that the gravitational pull exerted on an object by the Moon decreases with increasing distance to the Moon.

NOTE:

Ask the learners the following questions and discuss them as you are going through the activity.

Which part of the Earth and ocean is being pulled the most by the Moon?

The learner closest to the Moon (one of the "oceans").

Which part of the Earth and oceans is being pulled least by the Moon?

The learner farthest from the Moon (another "ocean".)

Ocean learner closest to the Moon: take three large steps towards the Moon.

Two ocean learners standing beside the Earth and the Earth learner: take two large steps toward the Moon.

Ocean learner farthest from the Moon: take one large step towards the Moon. Why have you moved towards the Moon by varying amounts?

Because the pull of the Moon depends upon the distance to the Moon.

Note what happens to the shape the "oceans" now make, are you still in a circle or forming an oval shape?

An oval.

NOTE:

Explain that the water from the oceans has "piled up" under the Moon and directly opposite the Moon. The two children standing beside "Earth" represent parts of the ocean where there is less water. You could ask the learners the following questions:

• Where are the oceans at the highest levels?

At the oceans nearest and farthest from the "Moon."

• Are the coastal areas next to those "oceans" seeing a high or low tide?

High.

• How many sides of the Earth experience high tide at the same time?

Two (in general).

• Which parts of the Earth are experiencing high tide right now?

The part that is under the piled up oceans.

• Where is low tide?

Near the oceans closest to Earth on either side.

Note which sides of "Earth's" body is experiencing high tide. (Front and back or left and right arms).

Front and back.

NOTE:

The formation of the second ocean bulge is simplified in this model and ignores subtle motions of the Earth.

Earth: spin around on the spot a few times stopping in a random position not directly facing the Moon. Remember that the Earth is continually spinning on its axis!

Note which sides of the "Earth's" body is experiencing high tide.

Learner-dependent answer. Should be the parts of the body directly facing towards and away from the Moon.

Now imagine that there is no Moon, but only the Sun to exert a gravitational pull on the Earth. Because the Sun is much farther than the Moon, its gravitational pull is only one third of the Moon's pull. The team member representing the Moon must now represent the Sun instead.

Sun learner: take an additional 10 steps away from the Earth so that you are 15 steps away in total.

All learners apart from the Sun: turn to face the "Sun". You are now ready to be pulled towards the Sun.

Ocean learner closest to the Sun: take one large step towards the Sun.

Two oceans learners standing beside the Earth and the Earth learner: take one normal step toward the Sun.

Ocean learner farthest from the Sun: take one small step towards the Sun.

Note what happens to the shape the "oceans" now make, are you still in a circle or forming an oval shape? How does the shape compare with that made when you were pulled by the Moon?

An oval but not as elongated as before, the "oceans" facing and directly opposite the Sun are closer to the Earth than they were when the Moon was pulling on them.

Ensure that learners can still find the sides of low and high tides even though they are less extreme.

QUESTIONS

How many sides of the Earth experience high tide at the same time?

Two

Where are they positioned in relation to the Moon?

Under the Moon and on the side of the Earth directly opposite the Moon.

As the Earth spins what happens to the position of high tides in relation to the Moon?

They remain under the Moon and directly opposite the Moon.

As the Earth spins what happens to the position of high and low tides on the surface of Earth?

The high and low tide are on different parts of the surface of Earth now.

Besides the Moon, what pulls on the Earth?

The Sun.

If there were no Moon, would we still have tides?

Yes but the difference between high and low tide would not be as extreme. Since the Sun is so far away, the Sun's gravitational pull would give the tides only a third of their height.

In this activity we have ignored the motion of the Moon revolving around the Earth. The time of high tide changes each day because the Moon is moving around the Earth. If we had no Moon, the tide due to the Sun alone would occur at essentially the same time every day. This activity also ignores the friction between the water and solid Earth as it spins, which causes the tidal bulges (piled up ocean bulges) to lie ahead of the Earth-Moon line in the direction of the Earth's rotation.

Look at the image below. It shows how the Moon's gravity distorts the shape of the Earth's oceans into an oval shape. Do you remember how the gravitational force depends on distance? The ocean on the side of the Earth closest to the Moon experiences a greater gravitational pull towards the Moon relative to the ocean on the far side of the Earth. This difference in gravitational pulls stretches the Earth's oceans into an oval shape. Along the Earth-Moon direction the oceans form two tidal bulges. At places in line with the Moon, where the oceans are experiencing a tidal bulge we have high tide. At locations which are at right angles to the Moon we have low tide.

As well as distorting the shape of the Earth's oceans, the Moon's gravitational pull also distorts the shape of the solid Earth. The solid Earth's bulge is about one hundred times smaller than the ocean bulge, but the Earth's crust closest to the Moon actually rises a few centimetres!

The highest tides in the world are at the Bay of Fundy in Canada. The bay is very narrow, so water rushing in from the ocean can rise and fall by up to 20 metres a day!

Why do you think there are two low tides and two high tides at a given beach per day? Look at the diagram above again. When the Moon is directly overhead your location you experience high tide. You also experience high tide when the Moon is directly opposite your location on Earth. Remember that the Earth spins on its axis once every 24 hours and so during one day you experience two high tides at a given location: one when the Moon is directly above your location and one when the Moon is directly opposite your location roughly twelve hours later. Similarly there are two low tides per day. This cycle continues as the Earth spins.

The actual times of low and high tides at a particular place on Earth are influenced not only by the Earth's spin but also by the Moon's motion around the Earth in its orbit. As the Earth spins, the Moon also travels around the Earth. The Moon rises about an hour later each day, and high (and low) tides also occur roughly an hour later each day.

The height of the tides varies slightly with the phase of the Moon. This is not because the gravitational pull of the Moon is changing: the Moon has the same amount of mass and therefore exerts the same gravitational pull at all phases. Rather, the change in the heights is due to the relative alignment of the Sun and the Moon. Let's look at this further in the following activity.

An animation demonstrating the tides.http://www.onr.navy.mil/Focus/ocean/motion/tides1.htm

# Spring and neap tides

INSTRUCTIONS

1. Look carefully at the following diagrams, it shows the size of the tides at Full and New Moon (top) and at the first and third quarter phase (bottom).

QUESTIONS

When the Sun, Moon and Earth are in a straight line the Sun's gravitational pull adds to the Moon's gravitational pull. What Moon phases does this correspond to?

New and Full Moon.

During what phases of the Moon do the Moon's and Sun's gravitational pulls partly cancel each other out?

First and third quarter.

During what Moon phases would you expect the highest high tides and the lowest low tides?

New and full Moon.

When the Sun, Moon, and Earth are lined up in a straight line (at the time of New or Full Moon), the pull of the Sun's gravity adds to the pull of the Moon's gravity creating extra-high high tides, and very low low tides. The difference in height between low and high tide is at its maximum at this time. These are called spring tides. When the Sun and Moon are at right angles to each other (during first and third quarter), the Sun's gravitational pull partially cancels out the Moon's gravitational pull and produces less extreme tides. The difference in height between the low and high tide is at its minimum at this time. These are called neap tides. Overall the Moon contribution to the Earth's tides is bigger than the Sun's contribution because it is much closer to Earth. If there were no Moon, the Earth's tides would be about a third of their current height.

Some additional activities are explained below.

Activity: Tides Poster

In this art and craft activity learners will make a poster to show the relative positions of the Sun and Moon producing spring and neap tides. You can use whatever art materials you have available, e.g. paint or pencils/crayons or felt pens. Ensure that you have blue, yellow and white and black colours so that the Earth's oceans, Sun and Moon can be accurately represented. Ensure that learners correctly identify which Earth-Moon-Sun alignments and Moon phases cause spring and neap tides . Ensure that learners draw the tidal bulge in the correct orientation and that the heights of their spring tides are higher at high tide and lower at low tide than their neap counterparts. Ensure that they correctly label places on Earth experiencing high and low tide.

MATERIALS:

• A3 sized poster paper
• paints, pencils or felt pens

INSTRUCTIONS:

1. Draw a poster showing the alignments of the Earth, Sun and Moon for both spring tides and neap tides.
2. Label the Sun, Earth and Moon, including the Moon phase.
3. Draw and label the tidal bulge created by the Moon.
4. Label where high and low tide occur.

Activity: Make a tide wheel

This is a very simple and fun activity for learners where they make a tide wheel using a template. Internet access and access to a printer is needed for this activity. The template used in the activity can be freely downloaded from the US weather service website at

http://www.srh.weather.gov/srh/jetstream/ocean/images/tidewheel_w.pdf

The template file contains two pages. If available print out the template file onto thin card, this will make the tide wheel sturdier, however normal A4 paper is also fine (the learners will have to be careful not to rip the paper in this case). The printed pages will have to be secured together in the middle using a brass fastener (sometimes called a split pin). These are available from stationary stores.

Learners can use their tide wheels to see what phases of the Moon cause spring and neap tides, and to investigate the relative contribution of the Sun and the Moon to the tidal height.

MATERIALS:

• one tide wheel template per learner
• one brass fastener per learner
• scissors

INSTRUCTIONS:

1. Cut out the two pages of the template along the dotted lines at the edge.
2. Cut out the areas marked "cut out" along the dotted lines to leave "windows" in your first sheet of paper.
3. Place the sheet of paper with the windows cut out on top of the second sheet of paper. Both pieces of paper should have their pictures facing up.
4. Align the sheets so that the black dots in the middle lie directly on top of each other.
5. Use your brass fastener to secure the two pieces of paper together at the black dot in the middle. You should still be able to rotate the bottom or top sheet around.
6. Move the top circle of card on your tide wheel and notice how the tides change with the position of the Moon.

QUESTIONS:

1. During New Moon, how are the Sun, Moon and Earth aligned?

They are in a straight line: Sun-Moon-Earth.

1. Is the tidal pull contribution from the Sun greater at Full Moon or First Quarter?

The tidal pull contribution from the Sun is greater at F ull Mo on.

1. What Moon phases correspond to neap tide?

Neap tides occur at First quarter and Third Quarter (where the orange colour lines up with the neap tide marker on the front of the tide wheel).

[Did you know] The Moon's orbit is gradually increasing, and the Moon is slowly moving away from the Earth. Due to this the tides used to be much higher than they are today, and they will continue to become smaller.

You can now see how important our closest neighbour the Moon is. The Moon's gravitational pull is responsible for the ocean tides!

### The effects of tides on shoreline ecosystems

This section covers the effects that the tides have on marine life living in the region between low and high tide: the intertidal zone. This is a particularly harsh environment for marine animals and many have developed unique adaptations to help them thrive in their shoreline environments. A good way to introduce this topic is to ask learners about their experiences at the beach, in particular in rocky pool areas. You could ask them what sort of marine life they have seen for themselves in these areas. If learners have not visited the beach themselves you can also show colourful photographs of marine life found in the intertidal zone and explain how different animals have adapted to live in the shoreline environment.

This section links nicely with the biodiversity areas of the Life and Living strand in Gr. 7 and in particular with the Interactions and interdependence within the environment section of the Life and Living strand in Gr. 8. A class visit to an aquarium would be an ideal excursion for this section, allowing learners to see for themselves the organisms and conditions described in this section.

• ecosystem
• intertidal zone

The region of the beach between high tide and low tide levels is called the intertidal zone. The intertidal zone is a harsh environment for marine animals to live. During storms the surf can be very rough and plants and animals must be able to withstand the battering from big waves and not get washed away! Animals and plants that live here are underwater at high tide but are exposed to the air during low tide. Some organisms may stay underwater if they are in small rock pools which do not empty out when the tide goes out. Those that are exposed to air at low tide, face hot temperatures in summer and cold temperatures in winter so they must be able to adapt to different temperatures.

Animals exposed to the air at low tide may be soaked in fresh water when it rains and yet be soaked in salty sea water at high tide. Therefore, they must also be able to adapt to different salt concentrations as the tides come in and out.

Different animals have adapted to this tough environment in different ways. For example:

# The effects of tides on shoreline ecosystems

@TN

This is an activity in which learners investigate what adaptions different shoreline animals have made in order to survive in the intertidal zone and write a summary of their findings. You can ask learners to look online for images and examples if they have access to the internet. Alternatively they can consult the school or local library or you can provide them with specific examples. Some images have been provided here as a starting point. This can also be used as a research task.

MATERIALS:

• Pictures and texts about shoreline animals. (Can be textbooks, library books or online materials as directed by your teacher).

Sea anemones look like plants with flowers but they are actually animals. Their tentacles contain a poison which paralyses their food (small fish and shrimps) when touched.

INSTRUCTIONS:

Study the pictures and texts and write a summary about how two different organisms are adapted to living in the intertidal zone. You can use the internet or other resources to do some more research.

Learners can use any of the examples given in this activity or those that they have read about. Answers could include animals that avoid drying out by burrowing, or closing their shells or plants that are covered in mucus. Answers could also include animals and plants that avoid being washed away by having strong "feet" that suck onto rocks.

High up in the intertidal zone water splashes only during high tide and the rest of the time it is dry. As you go lower down the intertidal zone, down the beach towards the sea, it gets progressively wetter for longer periods of time.

Some extra information about the different animals found in different areas in the intertidal zone:

High up in the intertidal zone (closer to the beach) the area is pounded by strong waves. Animals that live here need to be able to cling tightly to rocks to avoid being swept out to sea. Barnacles, limpets, periwinkles, and whelks cling tightly to rocks to avoid being swept out to sea. Seals and sea otters either rest or sleep above the intertidal zone so that they are not washed away or hit by the waves. If the tide comes in really high, they will move to another shoreline.

In the middle of the intertidal zone, tide pools often form and animals come to the tide pools to feed. Animals that live here can have softer bodies as this region is not so heavily pounded with waves. Sea anemones, snails, hermit crabs and starfish live in tidal pools.

In the lowest region of the intertidal zone where it is mainly wet, organisms are not well adapted to long periods of dryness. Some of the creatures found here are sea anemones, brown seaweed, crabs, green algae, limpets, mussels, sea slugs, starfish, sea urchins, shrimp, snails and sponges.

Marine life in the intertidal zone have to adapt to the rise and fall of sea levels at the beach. But marine life is not the only kind of life that has to take note of the tides. Many people also use the low tide to collect seaweed. Seaweed has many uses, including being a food source for people. In some cultures seaweed is used for medicinal purposes and to make various woven products, such as rope, baskets and mats.

Fishermen looking for big catches time their fishing activities according to the tides too. Lets investigate this further.

# How good a fisherman are you?

In this activity learners have to use a tide table to predict the best fishing times in Durban on a particular day. This emphasises how learning about the tides in the classroom has real world applications.

BACKGROUND:

Fish are easier to catch at times when they are feeding. The tides determine when most fish feed. When the tide is coming in or going out the moving water stimulates feeding. The fastest part of the tide is normally around two hours before and after low and high tides. These times are the best times to go fishing.

INSTRUCTIONS:

1. Look at the example tide table data for one day below and answer the following questions.

Durban - Thursday 29th August 2013

 Time Tide Height (m) Comment 00:56 Moonrise 02:29 0.85 Low tide 06:14 Sunrise 08:41 1.26 High tide 11:42 Moonset 14:52 0.93 Low tide 17:39 Sunset 21:34 1.27 High tide

QUESTIONS:

Thembela wants to go fishing at the best time around the first low tide of the day. What times could she go fishing?

00:29 or 04:29

Josh wants to go fishing while the Sun has set. What would be the best possible times for him to choose from?

00:29, 04:29, 19:34 or 23:34

Faried wants to go fishing while the Sun is up. What would be the best possible times for him to choose from?

06:41, 10:41, 12:52, 16:52

## Summary

• The Moon orbits the Earth once every 27.3 days. The Moon also spins on its own axis once every 27.3 days. Due to both these time periods being the same, we only ever see one side of the Moon from Earth.
• Gravity is a force that acts between all objects with mass. The size of the force acting on the objects is proportional to their masses and inversely proportional to their distance from each other.
• The Earth's gravity is responsible for holding the Moon in orbit around the Earth.
• The Moon's gravitational pull is mainly responsible for the tides on Earth.
• Neap tides occur when the Sun and Moon are at 90 degrees to each other.
• Spring tides occur when the Sun and Moon are in line with each other.
• The rise and fall of the tides affects marine life living along shorelines. They have adapted to this harsh environment in many ways to prevent themselves from drying out and from being washed away by strong waves.

Concept map

Complete the concept map by filling in the blank spaces. You can do this by reading the sentence that is made in the concept map. For example, "Gravity depends on mass of objects. If objects same distance apart, then ….........., exerts a stronger pull." What would the answer be? A "bigger object" or a "smaller object"? Fill the answer in. Also do this for the distance between objects. Would "closer objects", or "further away objects" exert a stronger pull? Then give a description of tides.

# Revision questions

Why do we only see one side of the Moon from Earth? [2 marks]

We only see one side of the Moon because the Moon rotates on its own axis at the same rate as it revolves around the Earth (27.3 days). Therefore the same half of the Moon always faces the Earth.

What is gravity? [1 mark]

Gravity is the force of attraction between two objects with mass.

What holds the Moon in orbit around the Earth? [1 mark]

The pull of gravity.

How does the gravitational force of attraction between two objects depend on their masses? [2 marks]

The larger the masses of the objects, the larger the gravitational force of attraction between them (at a fixed distance).

How does the gravitational force of attraction between two objects depend upon their distance? [2 marks]

The greater the distance between two objects with mass, the smaller the gravitational force of attraction between them (for fixed masses).

If you were to stand on the surface of the Moon you would experience only 1/6th the strength of gravity that you experience standing on the surface of the Earth. Why is this? [2 marks]

This is because the Moon is less massive that the Earth and so it has less gravity.

What causes tides? [2 marks]

Pull of the Moon's gravity on the Earth's oceans. And to a lesser extent the Sun's gravity.

Look at the following photo of boats on the sand. Do you think it is a problem that they are stuck on the sand? How will people get them into the sea?

It is not a problem as it is low tide at the moment so the water has receded and the boats rest on the sand. But, when it is high tide again the water will come up and lift the boats off the sand and people will be able to get them out to sea.

What kind of tides occur when the Moon is inline with the Sun? [1 mark]

Spring tides.

What kind of tides occur when the Sun, Earth and Moon are at right angles to each other? [1 mark]

Neap tides.

At what phases of the Moon do spring tides occur? [2 marks]

New Moon and Full Moon.

At what phases of the Moon do neap tides occur? [2 marks]

First Quarter and Third Quarter.

What would happen to the height of the tides if there were no Moon? [1 mark]

They would be only a third of the height that they presently are.

Draw a diagram to show the alignment of the Sun, Earth and Moon during neap and spring tides. [4 marks]

You can use the image from the activity on spring and neap tides as a reference for what learners should draw.

Explain why spring tides are more extreme than neap tides. [2 marks]

When the Sun, Moon, and Earth are lined up in a straight line the pull of the Sun's gravity adds to the pull of the Moon's gravity creating spring tides. When the Sun and Moon are at right angles to each other the Sun's gravitational pull partially cancels out the Moon's gravitational pull and produces less extreme tides. These are called neap tides.

Look at the following photo and answer the questions.

1. Do you think it is low or high tide? Give a reason for your answer. [2 marks]

2. What is the name given to this zone on the shoreline where the tides move back and forth? [1 mark]

3. What are the main risks to marine life living in this region? [2 mark]

4. How is the seaweed adapted to not dry out? [1 mark]

5. What other types of animals do you think you would find in this region? Give 4 examples. [2 marks]

1. It is low tide as there are rocks exposed which are normally underwater as they have seaweed growing on them.

2. The intertidal zone.

3. Drying out, damage by strong wave action and predation.

4. The seaweed is covered in a slimy layer which prevents it from drying out. It also clumps together.

5. Learners can list any of the animals included in this chapter, or else others which they may know about.

Total [33 marks]