Forces and Energy
Every time you draw back a bow, lift a bag onto a bench, or rub your hands together on a cold morning, a force is quietly moving energy from one place to another.
Printable Worksheets
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Q1 · You stretch an elastic band and hold it still, then let go and it flicks across the room. Where do you think the flicking energy came from?
Q2 · After cycling downhill you squeeze the brakes hard and they feel hot. Where did the heat come from?
● Know
- That a force can transfer energy to an object and make it move
- That stretching, compressing and lifting can store energy
- That friction transfers movement energy into heat
● Understand
- Why "doing work" means a force causes movement and transfers energy
- How stored energy (potential) can change into movement energy (kinetic)
- That energy is conserved, forces transfer it but never create or destroy it
● Can do
- Trace energy through a simple force situation from start to finish
- Identify where energy is stored and where it is transferred
- Explain an everyday example using force and energy correctly
A force on its own is just a push or a pull. The interesting thing is what a force can do: it can move energy from one object to another. When a force makes something move, energy is transferred to that object, and the moving object gains kinetic (movement) energy.
Scientists have a special word for this. When a force causes an object to move, we say the force does work. Doing work always means the same thing:
- A force pushes or pulls on an object
- The object moves in the direction of the force
- Energy is transferred to the object
Think of kicking a stationary soccer ball. Your foot pushes the ball (a force), the ball moves off (it speeds up), and energy has passed from you into the ball. The ball now has kinetic energy, which it did not have before. The harder you kick and the further your foot pushes, the more energy you transfer and the faster the ball travels.
If a force does not make an object move, no energy is transferred to it. Pushing as hard as you like on a brick wall that will not budge transfers no movement energy to the wall, even though you feel tired.
Sometimes a force does not send an object flying straight away. Instead, the force stores energy so it can be released later. This stored energy is called potential energy, and there are two important kinds at Year 7 level.
Elastic potential energy. When you stretch a spring, pull back an elastic band, or draw a bow, you apply a force that changes the object's shape. That force stores elastic potential energy in the stretched or squashed object. The moment you let go, the stored energy is transferred to movement, the band flicks, the arrow flies, the spring jumps. Compressing something, like pressing down a spring in a pen, stores energy the same way.
Gravitational potential energy. Lifting an object means pushing it upward against the force of gravity. As you lift, energy is transferred into the object and stored as gravitational potential energy. The higher you lift it, the more energy is stored. If you let the object fall, gravity pulls it down and that stored energy is transferred into movement, the falling object speeds up and gains kinetic energy.
Australian context: at a skate park in Sydney, a rider who pumps up to the top of a ramp stores gravitational potential energy. Rolling back down, that stored energy becomes movement energy and the rider speeds up, exactly like a ball dropped from a shelf.
Not all energy transfers leave an object moving faster. Friction is a force that acts whenever two surfaces rub together, and it transfers movement energy into heat, plus a little sound. This is why:
- Rubbing your hands together quickly warms them up, the movement energy of your hands turns into heat
- Bicycle and car brakes get hot, the moving energy of the wheels is turned into heat by friction
- A slide left in the sun feels warm, and sliding down it slows you as friction takes movement energy away
This brings us to one of the biggest ideas in all of science. Energy is conserved. This means energy can never be created out of nothing, and it can never be destroyed. Forces simply transfer energy from one object to another, or transform it from one kind into another.
Trace the campsite hook from the start of the lesson. Your muscles use chemical energy to move your hands. The friction between your hands turns that movement energy into heat. No energy vanished, it just changed form. The same is true for the slingshot: chemical energy in your muscles becomes elastic potential energy in the stretched band, then movement energy in the flying stone, and finally heat and sound when the stone lands. The total amount of energy stays the same at every step.
| Situation | Force involved | Energy change |
|---|---|---|
| Kicking a still ball | Push from your foot | Energy transferred to ball as movement (kinetic) energy |
| Stretching a bow | Pull on the string | Energy stored as elastic potential energy |
| Lifting a bag onto a bench | Lift against gravity | Energy stored as gravitational potential energy |
| Squeezing the brakes | Friction at the brake pads | Movement energy turned into heat and sound |
A child pulls a toy car backward, which winds up a spring inside it, then lets go and the car races forward across the floor until it slowly stops. Predict how the energy changes from start to finish. Where is energy stored, where does it become movement, and where does it finally go?
How close was your prediction?
Well done, you traced the energy from stored to moving to heat.
Key insight: forces move energy between stores, friction is where movement energy ends up as heat.
Below are 8 everyday situations. For each one, write where the energy starts (for example chemical energy in muscles, stored elastic energy, stored gravitational energy) and where it ends up (for example movement, heat, sound).
| # | Situation | Energy starts as | Energy ends up as |
|---|---|---|---|
| 1 | You kick a still soccer ball | ||
| 2 | You stretch and release a slingshot | ||
| 3 | You lift a backpack onto a high shelf | ||
| 4 | A dropped apple falls to the ground | ||
| 5 | You rub your hands together fast | ||
| 6 | A cyclist squeezes the brakes to stop | ||
| 7 | A wind-up toy car races forward | ||
| 8 | You draw back and release a bow |
Draw a simple three-stage cartoon strip of a ball being dropped from a high shelf onto the floor. Your strip should show:
- Stage 1, the ball held high on the shelf, labelled with gravitational potential energy stored
- Stage 2, the ball halfway down with a downward arrow, labelled gravity does work, stored energy becoming movement energy
- Stage 3, the ball hitting the floor, labelled movement energy turns into a small thud of heat and sound
After drawing (in your book or on paper), answer this question here, using the words force, energy and conserved:
Q1. Explain what it means in science to say that a force does "work" on an object. Give one everyday example. (2 marks)
Q2. A skateboarder rolls up to the top of a ramp, pauses, then rolls back down. Describe how the energy changes from the bottom, to the top, and back to the bottom. (3 marks)
Q3. A student says, "When my toy car runs out and stops, its movement energy has been destroyed." Explain why this is not correct, using the idea that energy is conserved. (3 marks)
Answers
▾MCQ 1
B A force does work when it causes an object to move and transfers energy to it. If nothing moves, no energy is transferred to the object, so pushing on a wall that will not budge does no work in the scientific sense.
MCQ 2
A A stretched elastic band stores elastic potential energy. It is not moving, so it has no kinetic energy yet, and it has not turned into heat. The stored energy is released as movement only when you let go.
MCQ 3
C As the ball falls, gravity does work on it and its stored gravitational potential energy is transferred into kinetic (movement) energy, which is why it speeds up. No new energy is created, the energy just changes form.
MCQ 4
C Friction between the brake pads and the wheel turns the wheels' movement energy into heat, plus a little sound. That is why the brakes get hot. The energy is not lost, it has changed into heat.
MCQ 5
B Conservation of energy means energy is only ever transferred from one object to another or transformed from one kind to another. It is never created from nothing and never destroyed.
Short Answer 1
Model answer: A force does work when it pushes or pulls an object and makes it move, transferring energy to that object. For example, kicking a still soccer ball: your foot pushes the ball, the ball moves off, and energy is transferred to the ball so it gains movement (kinetic) energy.
Short Answer 2
Model answer: At the bottom the skateboarder has movement (kinetic) energy. Rolling up the ramp, that movement energy is transferred into stored gravitational potential energy, so at the top, where the rider pauses, the energy is mostly stored as gravitational potential energy. Rolling back down, gravity does work and the stored energy is transferred back into movement energy, so the rider speeds up again. (Some energy is also lost to heat through friction at each stage.)
Short Answer 3
Model answer: The student is not correct, because energy cannot be destroyed, it is conserved. As the toy car rolls, friction between its wheels, axle and the floor transfers the car's movement energy into heat (and a little sound). The movement energy did not disappear, it changed form and spread out into the surroundings as heat, so the total amount of energy stays the same.
At the start of this lesson you met the hook: warm hands from rubbing, and a stone flying off a slingshot. Now you know the science. The warmth came from friction turning your hands' movement energy into heat, and the stone's flying energy was stored as elastic potential energy in the stretched band before you let go.
How does your new understanding of force and energy compare to what you first thought? Pick one of the two examples and trace the energy from start to finish, then explain why no energy was created or destroyed.
- A force can transfer energy to an object. When a force causes movement it does work, and the moving object gains kinetic energy.
- Stretching or squashing stores elastic potential energy, and lifting against gravity stores gravitational potential energy, releasing either gives movement energy.
- Friction turns movement energy into heat, and energy is conserved, forces transfer or transform energy but never create or destroy it.