Grade by Grade
E. Earth and Space Systems · Conservation of Energy and Resources

Stored Energy: Potential Energy

E2.3 A1.2 A1.4About 40 minutes
Learning goals
  • I can explain that potential energy is stored energy.
  • I can describe gravitational, elastic and chemical potential energy.
  • I can explain how stored energy changes into motion in a device or system.
  • I can do a fair test to see how height changes stored energy.

Words to know

potential energy
Stored energy, waiting to be used.
gravitational potential energy
Energy stored in something because it is high up. The higher and heavier it is, the more energy it stores.
elastic potential energy
Energy stored in something that is stretched, squished, bent or twisted, like a spring or an elastic band.
chemical potential energy
Energy stored in food, fuels and batteries.
system
A group of parts that work together, like a roller coaster or a hydroelectric dam.
fair test
A test where you change only one thing and keep everything else the same.

Ready, set... go!

Pull an elastic band back on your fingers. Hold it. Nothing is moving, but you can feel that something is ready to happen. Let go (aim it at a wall, never at a person!) and it flies.

Where did the motion come from? It was stored in the stretched elastic. Stored energy is called potential energy. Potential means "could happen".

Three ways to store energy

1. Gravitational potential energy: stored by being high up. When you pull your toboggan to the top of a hill, you give it stored energy. The higher it is, the more energy it stores. A heavier object at the same height also stores more. When it goes down, the stored energy changes into motion energy.

2. Elastic potential energy: stored by stretching, squishing, bending or twisting. A stretched elastic band, a squished spring, a trampoline pushed down by your feet, a bent hockey stick during a slap shot, and a wound-up spring in a toy all store elastic energy. The more you stretch or squish it (without breaking it), the more energy it stores.

3. Chemical potential energy: stored in food, fuels and batteries. A sandwich, a log of firewood, the gasoline in a car and a battery all store chemical energy. It is released when food is digested, when fuel burns, or when a battery is connected in a circuit.

A wooden toboggan with a curled front resting at the top of a long snowy hill.
A toboggan at the top of a hill has gravitational potential energy. As it slides down, that stored energy changes into motion energy. AI-generated image
Top: most stored energyBottom: most motion energy
A ball at the top of a ramp has the most stored (gravitational potential) energy. As it rolls down, the stored energy changes into motion energy. At the bottom it has the most motion energy.

Stored energy in devices and systems

Many devices and systems store energy and then change it into motion or other forms when it is needed.

  • Pull-back toy car: pulling the car backward winds up a spring inside (elastic energy). When you let go, the spring unwinds and turns the wheels (motion).
  • Roller coaster: a motor uses electrical energy to pull the cars up the first big hill. At the top they have lots of gravitational potential energy. As they roll down, it changes into motion energy.
  • Hydroelectric dam: a dam holds a big lake of water high up. The water stores gravitational potential energy. When it is let through the dam, it falls, moves fast, and spins a turbine, which makes electricity. Ontario has hydroelectric stations at Niagara Falls and on many other rivers.
  • Flashlight: the batteries store chemical energy until you switch it on.
How a hydroelectric dam uses stored energyGWater held high up:stored (gravitational)potential energyturbine + generator:electrical energyfalling water:motion energypower linesnot to scale
Water high up behind the dam stores gravitational potential energy. As it falls through a pipe it moves fast and spins a turbine, and the generator (G) changes that motion into electrical energy.
ExampleType of stored energyWhat it changes into
Sled at the top of a hillgravitationalmotion as it slides down
Water behind a damgravitationalmotion, then electrical
Stretched elastic bandelasticmotion when it is let go
Wound-up toyelasticmotion (and sound)
Batterychemicalelectrical
Firewoodchemicalheat and light when it burns
Examples of stored (potential) energy.
Worked example

Let's trace the energy on a toboggan hill

Ava pulls her toboggan up a snowy hill, then slides down. Describe how energy is stored and changed.

  1. At the bottom: Ava's breakfast gave her chemical energy. Her muscles use it to walk up the hill pulling the toboggan (motion energy).
  2. Going up: as Ava and the toboggan get higher, they gain gravitational potential energy.
  3. At the top: they are as high as they get, so they have the most stored energy. They are not moving yet.
  4. Sliding down: the stored energy changes into motion energy. They go faster and faster.
  5. At the bottom: most of the stored energy has become motion. (Next lessons: some also turns into heat and the sound of swishing snow.)

Chemical (food) → motion (climbing) → gravitational potential energy (at the top) → motion (sliding down). A higher hill would give Ava more stored energy and a faster ride.

Just for fun

Riddle: Why did the toboggan love sitting at the top of the hill?

It felt like it had so much potential!

Hands-on activity

Ramp test: does height change stored energy?

You need: a long, flat piece of cardboard or a flat board (about 60 cm long), books that are all about the same thickness, a toy car, a tape measure or metre stick, masking tape, paper and a pencil.

  1. Question: Does a higher ramp give a toy car more energy?
  2. Predict: what will happen when the ramp is higher?
  3. Make a ramp: put one end of the board on 1 book. Put a piece of tape at the top as the start line.
  4. Hold the car at the start line and let go (don't push!). Measure how far it rolls from the bottom of the ramp. Do it 3 times and write down each distance.
  5. Repeat with 2 books and then 3 books.
  6. Fair test: keep the same car, the same start line, the same floor, and always let go without pushing. Change only the number of books.
  7. Conclude: what pattern did you see? Why?

What to look for: the higher the ramp, the farther the car usually rolls. At the top of a higher ramp, the car stores more gravitational potential energy, so it has more motion energy at the bottom. If one result is very different from the others, try it again.

Safety (A1.4): test on a clear floor where no one will trip. Pick up the car and books when you are done. Keep small toy parts away from young children.

Your turn

1

What is potential energy?

2

Two identical books: one on a low shelf, one on a high shelf. Which has more gravitational potential energy?

3

Name two things that store elastic potential energy.

4

Where is the stored energy in a flashlight?

5

Why is water stored high up behind a dam?

6

Which stores more energy: an elastic stretched 5 cm or the same elastic stretched 15 cm?

More practice: warm-up

1

Gravitational, elastic or chemical? A sled waiting at the top of a hill.

2

Gravitational, elastic or chemical? A squished spring.

3

Gravitational, elastic or chemical? A sandwich.

4

Gravitational, elastic or chemical? A stretched hair elastic.

5

Gravitational, elastic or chemical? A battery.

More practice: on your own

1

Sam's results: ramp on 1 book, car rolled 80 cm; 2 books, 150 cm; 3 books, 210 cm. What pattern do you see? Why?

2

Why should Sam let the car go without pushing it?

3

Explain how a pull-back toy car stores and uses energy.

4

A child and an adult sit on two sleds at the top of the same hill. Who has more gravitational potential energy? Why?

5

How does a roller coaster get its stored energy at the start of the ride?

Challenge

1

Describe how energy is stored and changed in a wind-up toy, starting from the person who winds it.

2

Why do engineers try to make the water fall from as high as possible at a hydroelectric station?

3

Lina wants to stretch an elastic band as far as possible to store the most energy. Is there a limit? What safety rule should she follow?

Check your understanding

Quiz

1.

Potential energy is...

2.

Three identical balls are at different heights. Which has the most gravitational potential energy?

3.

A squished spring stores...

4.

Chemical potential energy is stored in...

5.

In Sam's ramp test, which should he change?

6.

At a hydroelectric dam, the water high behind the dam has...

For parents and teachers

Big idea (E2.3): energy can be stored as potential energy (gravitational, elastic, chemical) and then transformed in a device or system, such as a pull-back car, a roller coaster or a hydroelectric dam. The ramp investigation practises the scientific experimentation process (A1.2) and safe practice (A1.4).

How to help: look for stored energy in daily life: the toboggan hill, a trampoline, a pull-back toy, the batteries in the remote. Ask "Where is the energy stored? What does it change into?"

Common difficulties:

  • Thinking an object at rest has no energy. Height, stretch and chemicals all store energy.
  • Pushing the car in the ramp test. Remind your child that a push adds energy and spoils the fair test.
  • Forgetting mass. At the same height, a heavier object stores more energy.

Safety: elastic bands should never be aimed at people or pets, and should not be stretched until they snap. Keep the test area clear so no one trips over the ramp.