# Potential and kinetic energy

Chapter overview

2 weeks

This chapter builds on the basic concept of energy. The chapter explains the difference between kinetic and potential energy. The law of conservation of energy is also introduced: Energy cannot be created or destroyed, but can be transferred from one part of the system to other parts. This is a crucial concept in Physics and it is important to make sure that the learners understand it.

We have also not made mention of the different "forms of energy" within this content. There is disagreement about what the "forms of energy" are, and how long the list should be or could be. The "forms of energy" language is a problem when teaching learners about energy. For example, learners can be asked to name the "form of energy" in various examples and often learners are just told which one and have to memorise the answer. They are disempowered by the question. Learners are still unable to work out what happens. Furthermore, not remembering the correct "form of energy" can cost them marks in a test, but remembering the "form" correctly does not add anything to their understanding of energy or systems.

What we must focus on is systems which have different parts that learners can examine. It is sufficient to say that the potential energy in a system becomes the kinetic energy of some part of the system. We can have energy in a system in two forms - it can be stored in the system (potential energy) and it can cause changes in the system (kinetic energy). Therefore, the key concepts to focus on within these sections are: potential and kinetic energy, systems, transfer of energy between parts of a system and the conservation of energy.

2.1 Potential energy (1.5 hour)

 Tasks Skills Recommendation Investigation: How can we make the foam cup move further? Planning investigation, doing investigation, hypothesising, identifying variables and controls, measuring, recording, drawing graphs, analysing Suggested Activity: Elastic bands Raising questions, carrying out instructions, measuring, recording, interpreting information CAPS suggested Activity: Reading a cereal box Observing, comparing, interpreting information, drawing graphs CAPS suggested

2.2 Kinetic energy (1 hour)

 Tasks Skills Recommendation Activity: Which objects have kinetic energy? Observing, comparing, sorting and classifying Suggested

2.3 Law of conservation of energy (0.5 hours)

2.4 Potential and kinetic energy in systems (3 hours)

 Tasks Skills Recommendation Activity: Identifying energy transfers in mechanical systems Observing, interpreting, identifying and classifying CAPS suggested Investigation: The energy transfers when boiling water Doing investigation, hypothesising, observing, identifying variables, recording, drawing graphs CAPS suggested Activity: An electric fan system Building a circuit, identifying components and features CAPS suggested Activity: Flow diagrams for energy transfers Identifying types of energy transfer, describing, drawing flow diagrams, communicating information CAPS suggested

• What is potential energy?
• What is kinetic energy?
• Where do we get energy from?
• How much energy do I need?
• Can energy be created or destroyed?
• What is a system?

Renewable and non-renewable sources are where we get our energy from but what types of energy do we find in the world?

All energy can be placed into two main groups:

1. Potential energy
2. Kinetic energy

So what are these different types of energy and what does it mean if an object has potential energy or kinetic energy? Let's investigate!

## Potential energy

Start this section by doing the investigations first and allowing the learners to draw their own conclusions. This will lead them to a better understanding of what energy is and what it can do rather than a verbal explanation. There are several activities which deal with potential energy. If you do not have enough time to do all of them, then choose at least one of them. You can base your choice on the resources that you have available at your school. Do not completely ignore the other activities, though. Take some time to talk through what the outcomes would have been and then you could ask the learners to answer the questions at home.

• joule
• potential energy
• system

Throughout our investigation and learning about the concepts surrounding energy, we will be talking about systems and how energy is transferred within a system. A system is a set of parts that work together as a whole. A change in one part of the system will affect the other parts. This will become more clear as we see some examples throughout this term.

We are going to find the difference between potential and kinetic energy. Look at the following diagram which shows a ramp with a marble rolling down into a foam cup. The marble will knock the cup and make it move.

When the marble is released, it rolls down the ramp and transfers some of its energy to the cup. This transfer of energy is what makes the cup move. But where did the marble get energy from? Do you think you can make the cup move more or less depending on how far up the ramp you start the marble? Let's do an investigation to find out.

# How can we make the foam cup move further?

It is important to emphasise the importance of independent and dependent variables. Take special care to explain the difference between the two variables. The independent variable is the variable that you chose to change while doing the experiment. The dependent variable is what result you measure in your experiment. Learners would have encountered variables in the previous strands by now. Each group will need a set up and they will roll the marble from different heights into the cup. The higher up the ramp the marble starts, the further the cup will move.

INVESTIGATIVE QUESTION: If we roll a marble down a ramp and into a cup, how does the starting position of the marble affect how far the cup moves?

VARIABLES:

What will we change when performing this investigation?

The height from which the marble is released to roll down the ramp. This is the independent variable because the learners are changing it to see how far the cup moves.

What will we be measuring in this investigation?

The distance that the cup moves. This would be the dependent variable because the distance depends on how high the marble was before it was released.

Which things must stay the same?

The size of the marble must stay the same.

HYPOTHESIS:

Write a hypothesis for this investigation. When you do this, you need to write what you expect to observe. It does not have to be the correct answer to the investigative question.

The hypothesis should mention how the distance the cup moves would change if the height of the marble changes. Here are two possible examples:

• "The higher the marble is on the ramp, the further the cup will move."
• "The higher the marble is on the ramp, the less the cup will move"

Both of the above hypotheses mention how the height of the marble is expected to affect the distance the cup moves.

MATERIALS AND APPARATUS:

• a styrofoam cup
• a marble
• a pair of scissors
• a ramp (this can be a plank of wood or stiff card)
• books or wooden blocks to prop up the ramp
• rulers

METHOD:

1. Work in groups of 3 or 4.
2. Cut a hole in the lip of the cup so that when you turn it over on a table, there is a hole which a marble can now fit through, as shown in the previous diagram.
3. Build the setup as shown in the following diagram. Place the cup upside down on the table surface. Place the ramp so that it ends at the hole in the cup. Prop up the ramp with blocks or books. You will adjust the height of the ramp using different books or wooden blocks. Otherwise you can just hold the top of the ramp at the specified height.
1. Practice rolling the marble down the ramp and into the cup. You can use two rulers to create a path down the ramp to guide the marble into the hole so that it does not roll off the side of the ramp. Or else you can bend the cardboard so that the marble rolls down the middle on the fold. You can also try a cardboard tube like a roller towel inner. You will need to practice to see what works best with the materials that you have available.
2. Once you have found the best way to do this, you can start the measurements.
3. First set up the ramp so that the top of the ramp is at a height of 5cm. Roll the marble from a height of 5 cm and then measure how far the styrofoam cup moves.
4. Next adjust the height of the ramp by increasing by 5 cm each time. Each time place the marble at the top of the ramp and roll it down, measuring how far the cup moves.

The height of the ramp at each step will depend on what you use to prop it up with. Try and get blocks or books of equal thickness so that at each step the height increases by the same amount.

1. Repeat the measurements until you have at least 6 recordings.
2. Record your measurements in the table and draw a graph with a line of best fit.

If you make bigger ramps, you can also perform more measurements at different positions up the ramp.

RESULTS AND OBSERVATIONS:

Record your results in this table.

 Height of marble up the ramp (cm) Distance the cup moves (cm)

Use the information in your table to draw a graph of the height of the marble up the ramp versus the distance the cup moves. Before you draw the graph, answer the following:

Which is the independent variable? This is the value which you changed in the investigation. The independent variable is written on the x-axis (horizontal axis).

The height of the marble on the ramp.

Which is the dependent variable? This is the variable you measured. The dependent variable is written on the y-axis (vertical axis).

The distance the cup moves.

The independent variable is plotted on the horizontal axis. In this example the scale could be increments of 5 cm. The dependent variable is always plotted on the y-axis.

CONCLUSION:

Write a conclusion for this investigation. Remember to refer to your graph and hypothesis when writing your conclusion.

The conclusion should mention that as the height of release increases, the distance that the cup moves increases.

Was your hypothesis shown to be true or false?

This answer will depend on what the learner wrote for their hypothesis. If their hypothesis stated that the greater the height, the further the cup moves, then their hypothesis is shown to be true. If their hypothesis stated that the greater the height, the less the cup moves then their hypothesis has been proved to be false.

When you hold the marble at a distance up the ramp, you are preventing it from rolling down the ramp. This means that the marble has the potential to roll down and knock the cup. So, YOU gave the marble potential energy by picking it up and holding it at the top of the ramp. When the marble hits the cup, the marble transfers energy to the cup which then moves. The cup then comes to a stop after a while. Do you have any guesses about why the cup stops after a while?

Discuss this with your class. The cup moves along the surface and it experiences friction as it rubs along the ground. This causes it to come to a standstill. Friction will only be covered in more detail in Gr. 8 when learners look at friction in terms of static electricity and how rubbing objects transfers electrons resulting in a charge. A small demonstration that learners can do to briefly look at friction is to rub their hands together and observe that their hands become warm. When surfaces in contact with each other move against each other, the friction between them transfers kinetic energy to heat.

Your investigation will have shown you that the greater the vertical height of the marble, the further the cup moved. This tells us that lifting the marble to a higher position means that it has more potential energy than if it was released from a lower position.

So the higher an object is above a surface, the more potential energy it has. Think of another example of picking up a brick, as shown in the diagram. Here we are looking at a system consisting of: the arm, the brick and the Earth that pulls on the brick.

When the brick was on the floor, it had no potential energy. But when it is lifted up, it has potential energy. Where did the potential energy come from?

The potential energy came from the arm which lifted it.

The boy now lets go of the brick and it falls down to the ground and makes a hole in the sand. What received the energy of the falling brick?

Discuss this with your learners. The answer is the sand which goes flying up. Ask your learners where is the energy now? The answer is the molecules of the sand are moving faster. A sensitive thermometer would show that the sand is a little warmer than it was. Also, the energy went into disturbing the air and your eardrums got that energy when you heard the bang.

Do you think the hole in the sand pit will be deeper if we drop the brick from a higher point? Why?

Yes it will be as it has more potential energy at a higher point so by the time it hits the ground it will be moving faster (more kinetic energy).

So what we have seen is that the energy is all still there within this system, but it is not easy to use anymore. The sand is warmer but we can not actually use that energy for anything because the temperature increase was so small. So the energy in the system has not been destroyed, but it is less available for us to use.

Let's look at another example of stored energy and energy transfers within a system.

# Elastic bands

Try to get elastic bands of equal length and thickness. It is the only variable which really needs to be kept constant. If you are running short on time you can leave out this activity. Rather just discuss the conclusions one would draw from such an activity.

We are going to be shooting match boxes with elastic bands by stretching the bands and releasing them to hit the matchbox. What are the parts making up this system?

The parts of the system are the stiff fingers, rubber band, matchbox, table.

What is the energy input into this system?

Stretching the elastic band so the movement of the fingers transfers potential energy to the elastic band which is now stretched.

Do you think there is a relationship between how far the matchbox travels and the energy that the hand puts in at each try? Let's find out.

MATERIALS:

• empty matchbox
• elastic band
• ruler

INSTRUCTIONS :

1. Place the empty match box on a desk, mark the spot with a piece of paper.
2. First, practice shooting the matchbox with the elastic band. Each time, place the elastic band and matchbox in the same starting position and distance from each other.
3. Once you feel comfortable doing this, stretch the elastic band by a different amount each time and measure how far the matchbox moves with each try.
4. Place a ruler next to your elastic band and first stretch it by a small amount. For example, if your elastic band is 5 cm long when held pulled tight, but not stretched, between your fingers, then stretch it to 8 cm.
5. Release the elastic band so that it hits the matchbox across the desk.
6. Measure the distance that the match box moves across the desk.
7. Record the distance in the table below.
8. Put the empty match box back in its original position on the desk.
9. Repeat the experiment several times but stretch the elastic band a bit more than before each time.

The distances moved will depend on the type of elastic bands used and how rough the surface of the desk is. What is important is that the learners see that the small stretch moves the match box the shortest distance and the largest stretch moves the matchbox the furthest.

Record your measurements in the following table.

 Elastic stretch (cm) Distance moved (cm)

QUESTIONS:

Does the distance moved by the matchbox increase or decrease as you stretch the elastic band more? State the relationship between these two measurements.

The distance the matchbox moves increases as you stretch the elastic band more.

What did you have to do in order to stretch the elastic band and keep it stretched?

The elastic band had to be pulled hard by the learners and they cannot let go if they want to keep it stretched.

Energy is transferred from the elastic band to the matchbox and the matchbox moves. But, then it comes to a stop after a while. Where did the matchbox transfer its energy to?

The matchbox transfers energy to the air and the table.

When the elastic band was stretched it gained potential energy. We know this because your hand had to do some work to stretch the elastic band, and now the elastic band can snap back and move the matchbox. The elastic band needs energy to make the matchbox move, and it got that energy from your hand.

The further we stretched the elastic, the further it could push the matchbox. This tells us that the more we stretch the elastic band, the more energy is transferred from the elastic band to the match box.

Energy transfers have taken place within this system: Energy is transferred from the hand, to the elastic band, to the matchbox, to the air and the table surface. The table ends up a little warmer than it was as it now has most of the energy and the air has the rest. The energy has not gone, but again it's not available to use.

So did you notice that both the marble and the elastic band had potential energy? But we didn't do the same thing to give them that energy. We lifted the marble but we stretched the elastic. This means that there is more than one way to give something potential energy. Potential energy is energy that is stored within a system.

Now that you understand a bit more about potential energy, can you think of some more examples of things which contain potential energy? Think in terms of things which have the potential or the ability to change something or make something move.

What about some of the fossil fuels that we discussed in the last chapter, such as coal and oil? Do you think these have potential energy? Yes they do. For example, coal is burned in power stations to generate electricity (you will learn more about this later on in the term). So, we can say the coal has stored energy which is used to generate electricity. Coal has potential energy. This is the same for other fuels as well.

Do you remember making electric circuits in Gr. 6 last year? Do you remember using batteries? The batteries are the source of energy for the circuit. The batteries store energy. In other words, they have potential energy.

Where do we get our energy from? As we learned in Life and Living, nutrition is one of the 7 life processes. We have to eat food. Food is the fuel for our bodies.

Have you ever had a look at all the small writing on food packaging? The information gives us nutritional information about the food. It also gives us the amount of energy stored in the food. Have you noticed that this is often given in joules? So what is a joule? How do we measure energy?

The joule was named after an English physicist, James PrescottJoule (1818-1889).

We can measure energy, just as we can measure the mass of an object or how fast a car is going. The mass of an object is given in grams or kilograms, the speed of a car is given in kilometers per hour (km/h). In the same way, energy is measured in joules. There are 1000 joules in a kilojoule.

The joule is a measure of energy. A food joule is not different to an electrical joule, nor different to a joule that heats water, nor a joule that comes from the Sun.

At this point it is important to note that the joule is a measure of energy. It is just a number that we calculate after a lot of careful measurements on changes in a system. A food joule is not different to an electrical joule, nor different to a joule that heats water, or a joule that comes from the Sun. It is important that learners realise that joules of energy from food are no different to joules of energy from Eskom. But, if we reinforce the concept of different "forms" of energy, then learners are given the reason to think that energy from food must be different to energy from Eskom, whereas it is not.

The main idea is to simplify the concepts around energy for learners by eliminating the long lists of "forms" of energy that appear in tests and rather shift the focus to systems, which have parts that learners can examine and understand. It is sufficient to say that the potential energy in a system becomes the kinetic energy of some part of the system.

Let's have a look at the energy content for some of the cereals that we eat for breakfast.

Encourage the learners to bring in old cereal boxes well in advance of doing this activity. It would be a good idea to find some extras to bring to class for those who forget. You can even photocopy some of the cereal boxes and keep them for the following year.

If you want to extend this exercise you could ask the learners to compare their cereal with the rest of the class. Draw a table on the board with the different cereals and their energies. Get the class to draw a bar graph comparing the energies in the different cereals.

MATERIALS:

• cereal box
• pair of scissors
• calculator

INSTRUCTIONS:

2. Answer the questions that follow.

Each learner's answers will depend on the type of cereal that they have chosen. Make sure that the learners have kept their cereal box so that you can check their answers against the box details.

QUESTIONS

Sedentary means that you lead an inactive lifestyle and do not do any exercise.

What is the amount of energy per 100 g for your cereal? Write your answer in kilojoules and in joules

Learner-dependent answer. For example, Oats contains 1528 kJ per 100 g. This is 1528 000 J.

The cereal boxes often indicate an amount per 100 g and then an amount per serving, which is normally less. What is the amount of energy per serving on your cereal box? Remember to include how many grams the serving is.

Learner-dependent answer. For example, Oats contains 611 kJ per 40 g serving. This is 611 000 J. Weetbix cereal contains 529 kJ (529 000 joules) of energy per serving.

Look at the following table which gives the recommended daily amount of energy for an individual depending on your age and level of activity. This is a guideline as to how much energy you should consume in food in one day.

 Gender Age (years) Sedentary (kJ) Moderately Active (kJ) Active (kJ) Female 9 - 13 14 - 18 8 000 8 500 8 000 - 9 000 8 500 - 10 000 8 500 - 9 500 9 500 - 10 500 Male 9 - 13 14 - 18 8 500 10 000 8 500 - 10 000 10 000 - 11 500 9 500 - 11 000 11 000 - 13 000

According to the table, what is the recommended daily amount of energy for your age and level of activity?

Learner-dependent answer. For example, a female learner who is 13 and moderately active needs between 8 000 and 9 000 kJ per day.

What percentage of your recommended daily energy is being supplied by one serving of your cereal? Show your calculations below.

Learner-dependent answer. Learners must choose the row which corresponds to their age and gender. They must then consider, honestly, how active they would consider themselves to be. They can then use their cereal to calculate the percentage.

Example calculation:

Lets assume we have a male, aged 15 who is very active. His recommended daily allowance (RDA) would be between 11 00 - 13 000 kJ.

A serving of Nestle Milo Cereal contains 477 kJ

The percentage of RDA = 477/13 000 x 100 = 3,7 %

An additional question to ask learners and have a class discussion is:

Based on the percentage worked out in question 3, do you think this is a good cereal to eat for breakfast? Why do you think it's a good/bad cereal for breakfast?

The learners should decide whether their calculated percentage is low or high. If it is low, they could decide that it is better to eat a high energy meal for breakfast and then lower energy meals throughout the day, or they might indicate that they are hoping to lose/maintain weight and so a low percentage is a good thing. If their calculated percentage is high, they could indicate that it is better to spread out their energy intake over the entire day rather than concentrate it in one meal.

The nutritional value of a cereal also does not only depend on how much energy it provides, but also what other nutritional ingredients it offers.

There are no incorrect answers to this question. It is based entirely on their own interpretation of their needs. This can be a very sensitive topic, don't spend too much time discussing weight loss programmes as this is not a weight counselling session and learners could get confused between healthy eating and excessive dieting.

Alternatively, ask several learners what the energy content is for their cereal and then ask which one provides the most potential energy and which one the least.

1. The following photograph shows the nutritional information on a box of cracker biscuits. Study it and then answer the questions that follow.

What is the energy content per 100g in joules?

The values given on the box are in kJ and kcal so learners must convert the values by multiplying by 1000. The answers are 1492 000 J (1492 kJ)

What is the mass of one biscuit?

The mass is 7.5 g, as the mass given on the box of 15 g is for 2 biscuits.

The nutritional information gives the serving size of 2 biscuits, but you want to know what the energy content will be if you only eat one biscuit. Write down the answer below.

Energy for one biscuit = 224/2 = 112 kJ per biscuit.

You now decide that you want to eat 5 biscuits. What is the energy content for this serving of 5 biscuits?

Energy content for 5 biscuits = 112 x 5 = 560 kJ.

Do you now see why we can say that food has potential energy? We need energy to make our bodies function. We get our energy from the food we eat. The molecules which make up our food have energy stored inside them. We eat the food and use the stored energy to move our muscles and perform all our bodily functions. This stored energy is potential energy.

## Kinetic energy

• kinetic energy
• transfer

Think back to the last activity where we used elastic bands to move matchboxes. The stretched elastic band had potential energy. When the elastic band was released, it moved and snapped back and then hit the matchbox and caused it to move. So what do we call this energy that the moving elastic band and moving matchbox have? We call it kinetic energy.

A short video about kinetic energy

Kinetic energy is the energy that an object or system has because it is moving.

# Which objects have kinetic energy?

INSTRUCTIONS:

1. Think about the definition of kinetic energy and decide which of the objects below have kinetic energy.

 Object Does it have kinetic energy? (Yes or no) Give a reason for your answer. A lady running. A bird in flight. A stop street sign.http://www.flickr.com/photos/icanchangethisright/3542372195/ A roller coaster. Two chairs. An apple. A helicopter.

 Object Does it have kinetic energy? (Yes or no) Give a reason for your answer. A lady running. Yes The lady is running and moving so she has kinetic energy. A bird in flight. Yes The bird is flying and moving in the air so it has kinetic energy. A stop street sign. No The sign does not move and so does not have kinetic energy. A roller coaster. Yes The roller coaster is moving and so has kinetic energy. Two chairs. No The two chairs are stationary and so do not have kinetic energy. An apple. No The apple is still on the surface - it is not moving so does not have kinetic energy. A helicopter. Yes The helicopter is flying and so it is has kinetic energy.

QUESTIONS:

Which bucket has more potential energy, the one sitting on the bottom step of a ladder, or the one sitting on the top step of the ladder?

The bucket at the top of the ladder has more gravitational potential energy than the one at the bottom.

Does a car travelling at 100 km/h or at 200 km/h have more kinetic energy?

The car travelling at 200 km/h has more kinetic energy than when it is travelling slower.

When the wind blows, it is actually the air particles moving. What type of energy do the air particles have? Why?

It is kinetic energy as the air particles are moving.

You have a bucket full of water and you are about to tip the water out. What type of energy does the water have at this point? Explain why.

It has potential energy as it has the potential to fall back down to the ground.

When you tip the water out and it falls to the ground, what type of energy does it have now?

Now it has kinetic energy as it is moving.

At this level in Gr. 7, it is acceptable to state that the water has kinetic energy as it falls down (as it is moving). However, the water also still has potential energy as it falls. This is because as the water falls, it loses potential energy and gains kinetic energy as potential energy is transferred to kinetic energy. The total energy within the system is equal to the potential energy plus the kinetic energy as energy is conserved.

What have we learnt so far?

• Potential energy is the energy that an object has because of its position in a system. In the brick activity, the brick had potential energy when it was lifted away from the surface of the Earth. The brick and the Earth attract each other so they are a system. The higher you lift the brick, the more potential energy you give it.
• We know that moving objects also have energy, we call the energy of moving objects kinetic energy.

But, we have also seen something else. Think again of the marble activity:

• The marble at the top of the ramp has potential energy.
• When the marble was released, it rolled down the ramp and knocked the cup causing it to move.
• The marble therefore transferred energy to the cup.

We also saw this in the match box activity:

• The stretched elastic band had potential energy.
• When the elastic band was released, it moved and snapped back and then hit the matchbox and caused it to move. This means that the match box now has energy.
• Energy was therefore transferred from the stretched elastic band to the matchbox.

So, the potential energy in the elastic band is not lost. It is transferred to the matchbox. This brings us to our next section.

## Law of conservation of energy

• law
• theory
• conservation

In CAPS, this section comes after the potential and kinetic energy in systems. However, it is more logical to first discuss how energy is conserved within systems and to then look at the examples of the systems in the next section.

Take time to make sure that the learners understand the difference between laws and theories. Scientific theories are subject to development and new ideas are being developed all the time. Learners should be encouraged to see science as a developing discipline and not a static set of ideas. However, the science knowledge that we teach at school level is not in doubt. Most of it has been tested and known since the 1800's. You are encouraged to tell your learners something of the arguments and confusion among the people who were the first to investigate this knowledge, and also that current science in the academic world is constantly evolving.

The Law of Conservation of Energy states that energy cannot be created or destroyed, it can only be transferred from transferred from one part of the system to other parts. This means that we keep recycling all the energy in the universe all the time!

A fun song about conservation of energy.

Why are we talking about laws in science? Did you think laws were just for lawyers? Well, you would be wrong. In science we talk about laws and theories.

Scientific laws predict what will happen in a particular situation. The law has been tested repeatedly (often) and the results do not change. A law does not explain why something happens, it just says what should happen. Theories explain how or why things happen. Theories are also tested over and over again to make sure that they are valid.

Scientific laws and theories are not set in stone, they are just the best explanation for how the world works based on the information we have now. Scientific knowledge is constantly growing and changing as new discoveries are made.

Now that we know about the Law of Conservation of Energy, this matches our own observation that the energy in the elastic and matchbox example was not lost, rather it was transferred from the elastic to the matchbox. We can say that the elastic band and matchbox form a system. This is also true for the marble and cup example. Remember, a system is made up of different parts that work together or affect each other. Let's now look at some more examples of how energy is transferred within systems.

A PhET simulation to explore energy systems and energy conservation. http://phet.colorado.edu/en/simulation/energy-forms-and-changes

## Potential and kinetic energy in systems

Remember, energy cannot be created or destroyed. It is transferred from one part of the system to other parts. When it is transferred it can be stored or used to make something move and so potential energy can be transferred to kinetic energy in a system.

A song about kinetic and potential energy

We can look at how energy is transferred within different systems to show that energy is conserved. There are many different types of systems that we can look at to see how energy is transferred through the systems.

### Mechanical systems

A mechanical system is one which is based on mechanical principles and the different parts interact in a mechanism. A mechanical system usually involves movement of some kind. It is often a group of simple machines working together.

Do you remember the elastic bands pushing the matchboxes? Do you think that was a system? You are right. It is a mechanical system. The hand, elastic band and matchbox all form part of a mechanical system. Your hand transfers potential energy to the elastic band, this is the input energy. The potential energy of the elastic band was transferred to the matchbox as kinetic energy. No energy was created or destroyed. We experienced the Law of Conservation of Energy without even realising it.

A video explaining how pulleys work.

Another simple example if a pulley and rope system, such as at a construction site where the builders want to lift heavy objects up to a higher floor. The construction worker will pull on the rope which goes up over a pulley and to lift the heavy object higher.

What is the input energy in this system?

The input energy is the movement (kinetic energy) of the worker's arms as he pulls on the rope

What are the different parts making up this mechanical system?

The different parts are the worker's arms pulling on the rope. the rope, the pulley, the heavy object and the surface of the Earth.

What is the input energy transferred to within this system?

The kinetic energy is transferred to potential energy as the object is raised higher.

A swing or a seesaw are examples of mechanical systems.

Did you realise that when you were swinging on the park swings that you were a part of a mechanical system? When you are at the top of the swing's arc, you and the swing have potential energy because the Earth is pulling you and you are going to start moving down. The potential energy becomes kinetic energy as you swing through the arc.

What about when you throw a ball up into the air? Do you think this is a mechanical system?

When you throw a ball upward it slows down as it moves upwards, stops for an instant and then speeds up as it falls back down to your hand. Your hand moves to throw the ball and transfers energy to the ball which allows it to move upwards. Does this also follow the Law of Conservation of Energy? Yes, it does. No energy was created or destroyed. The kinetic energy was transferred from your hand to the ball which then starts to move. As the ball moves upwards, kinetic energy is transferred to potential energy as it moves further away from the ground. As the ball moves back down again, the potential energy is transferred to kinetic energy.

What are the parts involved in this mechanical system?

The hand throwing and the ball are the parts involved.

What is the input energy in this mechanical system?

The kinetic energy of your moving arm and hand as you throw the ball.

When does the ball have the most potential energy?

It is important to note that potential energy is relative to a reference point. So, in this example, if we consider the ground to be the reference point, then if you throw a ball upwards and catch it again in your hand, it always has potential energy as it is above the ground. Therefore, the ball has most potential energy when it is at the top of the throw when it stops briefly, before coming back down to the ground.

When does the ball have kinetic energy?

The ball has kinetic energy as it is moving upwards and then falling back downwards.

Let's have a look at some more examples.

# Identifying energy transfers in mechanical systems

Learner might battle to do this at first, so you can go through one or two of the examples with them, and if possible, also perform the demonstration of bending the wire back and forth. The key is to first identify the parts that are involved and then how the energy is transferred from one part to another within the system.

MATERIALS:

• a piece of wire

We are first going to perform a simple demonstration to identify the energy transfers within mechanical systems. Take a length of wire and touch it to your lips. How does it feel?

Learners should note that the wire feels cold.

Then, bend the wire into a U-shape and bend it back and forth 10 times quickly. Now, feel the temperature again at the bend. How does it feel?

Learners should note that it feels warm.

This is an example of a mechanical system. We can describe the transfer of energy as the potential energy within your arms is transferred to kinetic energy as you move them back and forth. This is transferred to kinetic energy in the wire which is then transferred to the lips as heat.

INSTRUCTIONS:

1. Look at the following pictures of different mechanical systems.
2. Identify the different parts in the system and then how energy is transferred from one part to another. You can discuss this with your partner.
3. Then write a few sentences to describe the energy transfers within each system.

The girl uses the energy in her muscles and pulls her leg back. When her leg is at its highest point, what energy does it have?

It has potential energy.

As she swings her leg back down towards the ball, describe the transfer of energy.

The potential energy becomes kinetic energy.

When her foot hits the ball, and the ball moves off, describe the transfer of energy in the system.

The kinetic energy from her leg is transferred to the ball and makes the ball move. The ball now has kinetic energy.

The muscles in the cricketer's arm pull the cricket bat upward. Describe the transfer of energy.

This movement transfers potential energy to the bat.

Describe the transfer of energy as the bat swings down and then hits the moving ball.

As the bat swings down the potential energy becomes kinetic energy as the bat moves. When the bat hits the ball, it transfers the kinetic energy to the ball. The kinetic energy allows the ball to move through the air.

Now that you have had practice with the other examples, use the following space to describe the transfer of energy within the above system as a ruler is pulled back and then flicks a pellet across the room.

When the ruler is pulled back, the movement of the hand has kinetic energy which is transferred to the ruler. The ruler has gained potential energy and when it is released, the potential energy becomes kinetic energy as the ruler flicks backwards. The kinetic energy is transferred to the pellet and so the pellet moves across the room.

### Thermal systems

We will learn more about how particles behave next year when we look at the particle model of matter.

Did you know that the particles that make up a substance or object, such as atoms or molecules, also have kinetic energy? Particles which have more kinetic energy will move faster than particles which have less kinetic energy. When the particles are moving very fast, we feel the substance and say "That's hot!". This is because the temperature of a substance depends on the kinetic energy of the particles.

The thermal energy can be transferred from one object to another in a thermal system. When thermal energy is transferred, this is called heat. We will look more at this in the next chapter, but for now let's look at some simple examples of energy transfers within thermal systems (heating).

We will look more at heat as a form of energy transfer in the next chapter.

# The energy transfers when boiling water

This investigation works best with water placed in a beaker and heated over a Bunsen burner using a tripod. However, if you don't have a Bunsen burner you can use a candle and a tin can. The candle will not provide a large amount of heat energy and so you should use a small amount of water in order for it to reach boiling point within the lesson time. Remember to use an alcohol thermometer rather than a mercury thermometer.

Although this may seem like a very simple investigation, and learners have heated water before, the focus here is different in that we are investigating the energy transfers. This is also an opportunity for learners to practice recording, observing and translation skills, such as drawing a graph.

INVESTIGATIVE QUESTION:

What happens to the temperature of water when it is heated over a flame?

VARIABLES:

We will be measuring the change in the water temperature over time.

Which quantity/variable are you in control of? This is the independent variable.

Time is the independent variable

Which variable are you measuring in response to the independent variable? This is the dependent variable.

The temperature of the water is being measured.

Which variable are you keeping constant?

The amount/volume of the water must be kept constant

HYPOTHESIS:

Write a hypothesis for this investigation. (Hint: What do you think will happen to the temperature of the water. Will it go up or down?)

The hypothesis should mention how the dependent variable would change with a change in the independent variable and should mention which variables must remain constant. In this investigation a suitable hypothesis could read: 'The temperature of the water will increase as time increases if the amount of water is kept constant' or 'The temperature of the water will decrease as time increases if the amount of water is kept constant.'

Remember that a hypothesis doesn't have to be correct. It is a prediction made before any investigation is done and so the outcome is not necessarily known beforehand. Do not discourage learners from developing their own hypotheses. Emphasise that a hypothesis is just as valuable if it is rejected after the investigation.

MATERIALS AND APPARATUS

• 150 ml or 250 ml beaker
• tripod
• gauze
• Bunsen burner
• matches
• thermometer
• stopwatch
• retort stand
• clamp

If you don't have Bunsen burners you can use spirit burners or a candle.

METHOD :

1. Pour 200 ml of water into a beaker.
2. Place the beaker onto the wire gauze on the tripod.
3. Carefully place the thermometer into the water. When you take the readings, the thermometer should not be touching the sides of the beaker. Alternatively, if you have a retort stand and clamp, the thermometer can be clamped in the stand with the bulb in the water.
4. Light the Bunsen burner.
5. Measure the temperature of the water every 30 seconds until the water starts to boil.
6. Once the water starts to boil, take 3 to 5 more readings.
7. Write down your observations in the table.
8. Once finished, turn off the Bunsen burner and leave the beaker of water to stand.
9. Plot a graph showing the relationship between the time and the temperature.

Make sure that learners observe that the temperature remains constant once the water starts to boil. Once the learners have completed their measurements, turn off the Bunsen burner and leave the water to cool while they carry on with the rest of the task and questions. They will then have to observe what happened to the water once it was left to stand.

RESULTS AND OBSERVATIONS:

A table to record your observations:

 Time (seconds) Temperature ( o C) 30 60 90 120 150 180

If the water takes longer than to boil, ask the learners to add rows to the bottom of their tables. If it takes less, ignore the rest of the table. Each row must represent half a minute (30 seconds).

Use the following space to draw a line graph for your results.

First, think about what will go on your horizontal, x-axis? This is what you changed.

Time goes on the x-axis as this is the independent variable.

What will go on the vertical, y-axis? This is what you measured.

Temperature is the dependent variable.

NOTE:

Learners must provide a heading for their graph. for example "The change in water temperature over time". The graph must show data points with a line of best fit drawn through them. The graph must also flatten out at the end as the water boils.

The temperature of the water kept increasing, until it started to boil. What temperature did the water boil at?

This might vary slightly depending on your area and altitude, but it is around 100 oC.

What did you observe in the temperature when the water started to boil?

When the water reaches boiling point, the temperature remains constant while the water is changing state from a liquid to a gas.

The reason we do a boiling point curve like this is so that the learners can see that the curve flattens out and temperature remains constant. This is how we find the boiling point of a liquid. It is useful to do a boiling point curve of another liquid as well, such as Coca Cola or orange/apple juice to get a similar shaped curve so that learners understand that at boiling point of that liquid the temperature remains constant for while. If you leave it longer still the temperature will begin to fall as more water particles move into the atmosphere.

This links back to what was covered in the previous term in Matter and Materials in Chapter 1 on the Properties of Materials.

Do you remember that we learned about boiling and melting points last term in Matter and Materials in Properties of Materials? If you do not have your previous workbook with you, you can always visit the website atwww.curious.org.za

CONCLUSION:

What can you conclude from your results?

Learner-dependent answer. The learners should conclude that the longer you keep the water over the flame, the higher the temperature of the water, until it reaches boiling point. At boiling point, the temperature remains constant as the water is changing state from a liquid to a solid.

Can you accept or reject your hypothesis?

QUESTIONS:

In order for the water to boil, the thermal energy of the water must increase. Where do you think the energy came from to make the water boil?

The energy for the temperature increase came from the burning gas in the Bunsen burner.

Describe the transfer of energy within this thermal system as the water is heated.

The thermal energy (kinetic and potential energy) of the flame in the Bunsen burner/candle is transferred to the water. The thermal energy of the water therefore increases and the temperature rises.

NOTE: Remember that temperature is a measure of the average kinetic energy of the particles.

After the water has boiled, and you then turn off the Bunsen burner, what happened to the water in the beaker?

It cooled down.

NOTE: This is because once the Bunsen burner is switched off, no more energy is supplied to the system and so the loss of energy is greater than the gain and the temperature decreases.

What do you think happened to the thermal energy of the water? Describe the transfer of energy.

The thermal energy was then transferred from the water to the surrounding air.

1. A Gr. 7 learner is conducting the investigation and read the temperature off the thermometer as it is set up in the diagram below. What is wrong with this set-up? What is your advice to the learner?

The thermometer is resting on the bottom of the beaker and touching the side. This could give an inaccurate reading. The thermometer should either be held by the learner so that it does not touch the sides while they take the reading, or else clamped in a retort stand with the bulb in the water.

So, what have we discovered? The temperature of the water increased. This means that the water particles must have been given more kinetic energy. The energy must have come from the Bunsen burner flame. The flame is there because we are burning gas so the energy must have been stored in the gas. If it is stored energy then it is potential energy.

Build your own skate park with this simulation and see what happens to the potential, kinetic, and thermal energy of the skateboarder. http://phet.colorado.edu/en/simulation/energy-skate-park-basics

This is a fun simulation that learners can use to investigate potential and kinetic energy. There is the example in the introduction that can be used to show them how to use it, and then there is the track playground that allows you to create your own track. Click on the bar graph, or pie chart menu options for a real-time display of the fluctuations in potential, kinetic and thermal energy.

Once they have mastered the basics, and if there is more time you can show learners the more advanced skate park simulation. The graphs in this version do become slightly more involved but represent the features of the energy system beautifully. http://phet.colorado.edu/en/simulation/energy-skate-park

PhET tips for teachers are available here: http://phet.colorado.edu/files/teachers-guide/energy-skate-park-guide.pdf

So, we have discovered that the potential energy stored in the gas has been transferred to the water particles as kinetic energy. No energy has been created, it has been transferred from the gas to the water. The energy of the system has been conserved.

### Electrical systems

Do you think an electric circuit is a system? Look at the following image and discuss this with your partner. Write down whether you think it is a system or not and why.

An electric circuit is a system as it consists of different parts that do something, ie. make the light bulb glow. Ask learners to identify the different parts in this system. They are: the battery, the light bulb, the switch (paperclip) and the conducting wires.

What is the source of energy is this electric circuit? In other words, what is the input energy in this system?

The battery is the source of energy, potential energy is the input energy in this system.

What is the result of the energy transfer in the system? In other words, what is the energy output?

The output is that the light bulb lights up/glows.

Let's look at another example of an electrical circuit which makes a motor turn to see the different energy transfers within the system.

# An electric fan system

If possible, make this circuit in class with your learners so that they can observe the changes in the circuit and the movement of the fan. You can use any small device that rotates, such as a fan or a small motor. If you do have a small motor, you can attach a sucker stick to the rotating shaft with a piece of Prestik to make a fan.

MATERIALS:

• small electric fan or motor
• conducting wires
• battery
• switch

You can also make your own switch, as described below.

INSTRUCTIONS:

1. If possible, make the following circuit in class. However, if you do not make the actual circuit, study the image and answer the questions.
2. To make the circuit, attach a small fan or motor to a battery using the conducting wires.
3. Attach a switch in the circuit as shown in the image. You can make your own switch using a piece of board and pressing two metal pins into it. Then, bend a metal paper clip and attach it to the one drawing pin as shown below.
4. Close the switch and observe what happens to the fan.

QUESTIONS:

What are the parts making up this electrical system?

The battery, the fan/motor, the switch, the wires.

Which part of the system provides the input energy to the system?

The battery provides the input energy (potential energy).

What happens to the fan or motor when you close the switch?

The fan starts to turn/rotate/move.

What type of energy does the fan now have?

Kinetic energy.

1. Using your answers to the previous questions, complete the following flow diagram which describes the energy transfers within this electrical system. You need to fill in the type of energy at each step.

The following shows the completed diagram with the answers learners should supply:

The battery has potential energy which is transferred to the electrons in the circuit. The electrons have kinetic energy which they transfer to the motor. The motor uses the kinetic energy to turn. The turning motor turns the blades of the fan.

### Biological systems

Do you know that we also get biological systems? You have come across these types of systems before in Life and Living, but now we are going to talk about them in terms of how the energy is transferred within these systems, and conserved.

Do you remember learning about photosynthesis and food chains in Life and Living? This is an example of a biological system. Let's find out why.

A plant uses the radiant energy from the Sun to make its own food through the process of photosynthesis. The energy from the Sun is stored as potential energy in plants, mainly as starch. Have a look at the following image to remind you.

What process is being shown in the diagram? Write a sentence to describe the requirements for this process.

The diagram is showing photosynthesis. The plant uses water, carbon dioxide and sunlight energy to produce glucose and oxygen.

When an animal eats the plant it uses the potential energy in the food which is released during respiration. This is then used by the animal to move and for all its life processes. So the potential energy in the food which the animal eats is transferred to kinetic energy. Energy has been transferred from the Sun to the plant to the animal.

When we eat plants or animals we are able to use the stored potential energy to make our bodies function.

Is the energy conserved in a biological system? Yes, it is! The plants change the Sun's energy into potential energy which it stores inside itself. Animals then eat the plants and the stored potential energy is transferred to them. The animals use the stored energy to enable them to move. This means the potential energy within the animal has been transferred kinetic energy. As the animal moves and performs its functions, this kinetic energy is transferred to the surroundings. No energy has been created or destroyed, just transferred from the Sun to the plant to the animal.

Let's revise the energy transfers within some systems by studying and drawing flow diagrams.

# Flow diagrams for energy transfers

INSTRUCTIONS:

1. Study each of the following diagrams which show different systems.
2. Draw a flow diagram, similar to the one you did for the electric fan in the space provided.
3. Then write a few sentences underneath on the lines to describe how energy is transferred between the different parts in each of these systems.
4. The first one has been done for you.

This flow diagram describes the transfers of energy.

The tennis player's arm and racket have potential energy as they are raised. As the girl swings her arm, this potential energy is transferred to the tennis racket as kinetic energy. The tennis racket transfers energy to the ball as kinetic energy which enables the ball to move through the air.

QUESTIONS:

This is a food chain. Draw a diagram showing the energy transfers in this biological system.

An example of the flow diagram learners could produce:

Write a description of the energy transfers below.

The berries have potential energy in them. The bird eats the berries and this energy is transferred to the bird as potential energy. Most of the energy is used by the bird and transferred as kinetic energy as it moves around. The bird is then eaten by a cat and the potential energy in the bird's flesh is transferred to the cat as potential energy which is then transferred to kinetic energy as the cat moves around and performs its life processes.

NOTE: Although learners will only look at food chains and energy pyramids in more detail next year in Life and Living, this is an introduction to how not all the energy is transferred onto the cat as most of it is used by the bird as it moves around and performs its functions and processes.

Draw a diagram showing the energy transfers in this electrical system.

An example of the flow diagram learners could produce:

Write a description of the energy transfers below.

The battery/cell has potential energy which is transferred to kinetic energy in the alarm bell as the hammer moves back and forth to produce sound.

In the previous example showing the berries, the bird and the cat, we saw an example of a food chain. Do you remember learning about food chains in Gr. 6? A food chain only shows the transfer of energy between organisms, and does not include the Sun. So, it always starts with a producer. Is the image below an example of a food chain? Why or why not?

No, this is not a food chain, as a food chain only shows the transfers of energy between organisms, highlighting the feeding relationships. This diagram also includes the Sun, as well as showing the horse moving a cart.

We can rather call this an energy transfer sequence. Draw a flow diagram to explain the energy transfers in this biological and mechanical system.

An example of the flow diagram learners could produce:

Write a description of the energy transfers below.

The energy from the Sun is transferred to potential energy within the carrots as they photosynthesize and produce food. The horse then eats the carrots and this potential energy is transferred to potential energy within the horse. The horse then moves and pulls a cart, so the potential energy in the horse is transferred to kinetic energy in the horse and in the cart as it moves along.

Let's now look at a more complex system which involves many different parts working together. Do you remember learning about hydropower as an source? Is it renewable or non-renewable?

Renewable.

We will learn more about food chains and the interactions between organisms next year in Gr. 8 Life and Living.

1. Study the following diagram which shows a hydropower plant at the edge of a dam. Then answer the questions that follow.

The water in the dam on the left is high up. It has the ability to fall down. What kind of energy does the water have?

The water has potential energy.

As the water flows down the outlet from the dam, describe the transfer of energy.

The potential energy is transferred to kinetic energy as the water moves/flows down.

The flowing water then turns the turbine. This is a mechanical system. What energy does the turbine have?

It has kinetic energy.

The generator then transfers the energy between two systems. The kinetic energy in the mechanical system is transferred to kinetic energy in the electrical system as it generates electricity. What parts make up the electrical system in the diagram?

The electrical system is made up of the generator, the power lines and then the houses/buildings in the city.

What is the output from this whole system? In other words, what does the city get?

The city gets electricity to run appliances, machines, equipment, lights and heating systems.

• Potential energy is energy which is stored in a system.
• Kinetic energy is energy which an object has because it is moving.
• Energy is measured in joules (J).
• Energy cannot be created or destroyed. It can only be transferred from one part of a system to another. This is the Law of Conservation of Energy.
• Energy is transferred within systems. The input energy is transferred through the system and energy is conserved.
• There are various energy systems, such as:

• mechanical systems
• thermal systems
• electrical systems
• biological systems
• Energy is also transferred between different systems.

Concept map

Complete the concept map below by filling in some examples of objects with either potential energy or kinetic energy that you learned about in this chapter.

We will study the national electricity grid in more detail at the end of the term.

Teacher's version showing some examples of what learners could put in for potential and kinetic energy. There are many other examples too.

# Revision questions

What is potential energy? Give two examples of systems which have potential energy [3 marks]

It is the energy stored inside a system. There are many different examples of potential energy. Some examples are: objects which are held above a surface, elastic bands which have been stretched, batteries contain potential energy.

What is kinetic energy? Give two examples of systems which have kinetic energy? [3 marks]

It is the energy a system has because it is moving. There are many different examples of kinetic energy. Learners can use any moving object as an example.

What does the Law of Conservation of Energy state? [1 mark]

Energy cannot be created or destroyed. It is transferred from one part of a system to another.

Look at the picture below.

1. Which ball has the most potential energy? [1 mark]

2. Explain your choice. [1 mark]

1. Ball A has more potential energy.

2. Ball A is higher than ball B relative to the ground and so it has more stored energy.

Complete the sentences by filling in the missing words. Write the sentence out in full and underline your answers.

1. A plant receives energy from _____ and uses the energy to make _____. The plant then changes some of the sugar into _____ and stores it in leaves, fruit and other parts. The plant has _____ energy which you can get when you eat the plant. [4 marks]

2. When a plane carries skydivers high into the sky, it is giving them _____ energy. When they jump out and free-fall, they have _____ energy. [2 marks]

1. A plant receives energy from the Sun and uses the energy to make food/sugar/glucose. The plant then changes some of the sugar into starch and stores it in leaves, fruit and other parts. The plant has potential energy which you can get when you eat the plant.

2. After skydivers jump out of a helicopter or plane, potential energy is transferred to kinetic energy as they fall.

Draw an energy transfer flow diagram to show how energy gets from the Sun into your food and then to you. [3 marks]

Learners must draw a Sun then a plant then either a person, or another animal which eats the plant and then a person which eats the animal.

A high jumper starts running. As she approaches the bar, she pushes off the ground and lifts her body off the ground and flies over the bar. She then falls down into a large padding on the ground.

Think about her jumping, from the moment her feet leave the ground. She goes up in the air, she almost stops as she goes over the bar, and then she comes down again.

1. Where does she have the most potential energy? [1 mark]

2. Where does she have the most kinetic energy? [1 mark]

3. Does she have some potential energy and some kinetic energy at any point in her jump? If you say yes, name one point where it is true. [2 marks]

1. At the top of her jump (as this is when she is the highest above the ground).

2. She has the most kinetic energy just before she touches the ground/pad again (as this is where she will be moving the fastest).

3. Yes, she does have both kinetic energy and potential energy at points during her jump. They are on the way up / on the way down (either).

Which type of energy do each of the following systems contain (kinetic or potential or both types)? [6 marks]

1. A mountain biker at the top of the mountain.

2. Petrol in a storage tank.

3. A race-car travelling at its maximum speed.

4. Water flowing down a waterfall before it hits the pond below.

5. A spring in a pinball machine before it is released.

6. A running refrigerator motor.

1. Potential energy

2. Potential energy

3. Kinetic energy

4. Both

5. Potential energy

6. Kinetic energy

Study the following image and answer the questions.
1. There are two systems involved in this image of heating water in a kettle that is plugged in. What are they? [2 marks]

2. Describe the energy transfers within and between these two systems. [2 marks]

1. They are an electrical system and a thermal system.

2. The electric current transfers energy to the hot-wire in the kettle, which transfers energy to the water and so the water molecules get more and more kinetic energy until the water starts to boil.

Total [32 marks]