Kinetic Energy
In 2023, ANCAP crash-tested a 1,500 kg SUV at 64 km/h and found it carried 235,000 J of kinetic energy — enough to crumple steel in milliseconds and show why speed limits save lives.
Printable Worksheets
Print or save as PDF — or build a custom worksheet from any module's questions.
Q1 · A tennis ball and a bowling ball are thrown at the same speed. Which is harder to stop? What does this tell you about energy?
Q2 · Why are car crashes at 100 km/h so much more destructive than at 50 km/h — even though the speed only doubled?
● Know
- That kinetic energy (KE) is the energy of motion
- That KE depends on both mass and speed
- That doubling speed has a much greater effect on KE than doubling mass
● Understand
- Why faster speeds make collisions far more dangerous (squared effect)
- How KE converts to other energy forms when motion stops
- How KE is used in everyday Australian contexts (hydroelectric, wind, vehicles)
● Can do
- Identify objects with kinetic energy and explain why they have it
- Describe the energy transformations that occur in a crash or braking event
- Trace energy conversion chains from moving water to electrical energy
Kick a football gently and it rolls a few metres; belt it as hard as you can and it flies across a field. The moving ball has energy — and the harder you kicked it, the more energy it carries. Scientists call this kinetic energy, and it turns out it grows far faster than most people expect: a car at 100 km/h carries four times the kinetic energy of one at 50 km/h — not twice. That's why speeding is so much more dangerous than it looks.
Kinetic energy depends on two things:
- Mass — a heavier object moving at the same speed has more KE. A truck and a bicycle both at 60 km/h: the truck has massively more KE because of its mass.
- Speed — the effect is much bigger than for mass. When speed doubles, kinetic energy quadruples (the relationship is "squared"). When speed triples, kinetic energy becomes nine times larger.
| Speed change | Effect on KE | Example |
|---|---|---|
| Double the speed (×2) | KE becomes 4× larger | Car 50 → 100 km/h: KE ×4 |
| Triple the speed (×3) | KE becomes 9× larger | Car 50 → 150 km/h: KE ×9 |
| Double the mass (×2) | KE becomes 2× larger | 2 kg ball vs 1 kg ball at same speed: KE ×2 |
This "squared" effect explains why speed cameras are set where they are, and why Australian Road Rules allow no more than 10 km/h over a speed limit. Just 10 km/h extra nearly doubles stopping distance at highway speeds.
When a moving object stops suddenly — a car crash, a ball hitting a wall — all of its kinetic energy must go somewhere. It converts to:
- Deformation energy — the car crumples, the ball squashes. The crumple zone in a modern car is designed to absorb KE by bending.
- Thermal energy (heat) — brake pads heat up, tyres get warm.
- Sound energy — the bang, squeal or crunch.
Australian road safety researchers at the Transport Accident Commission (TAC) and Transport for NSW use the KE-speed relationship to set speed limits. At 60 km/h, a pedestrian hit by a car has about an 85% chance of survival. At 80 km/h, that chance drops to around 15%.
Car safety features all work on the same principle — spreading the KE transfer over more time reduces the peak force:
- Crumple zones — controlled deformation absorbs KE gradually.
- Airbags — soft cushion slows the passenger over more time.
- Seatbelts — keep passengers in the car and spread the force across the chest.
Kinetic energy isn't just about vehicles — it powers much of Australia's electricity and shapes the natural world.
- Snowy Mountains Hydroelectric Scheme — water stored in Lake Eucumbene has gravitational potential energy. As it flows downhill through tunnels, it gains kinetic energy. That KE spins massive turbines → electrical energy. The Snowy Scheme supplies about 4 500 MW of generating capacity.
- Wind farms — moving air molecules carry kinetic energy. Wind turbine blades capture it and convert it to electrical energy. The Hornsdale Wind Farm in South Australia (paired with the world-famous Hornsdale Power Reserve battery) shows how wind KE can be stored and dispatched on demand.
- Olympic sprinters — a sprinter converts chemical energy (food/glucose) into kinetic energy. At peak speed, an elite sprinter may be moving at over 10 m/s, carrying significant KE in their body mass.
- Molecules and temperature — at the particle scale, the molecules in warm air are moving faster than those in cold air. Their kinetic energy IS thermal energy. Higher temperature = faster moving molecules = more molecular KE.
A car is travelling at 100 km/h and needs to brake suddenly. If the same car had been travelling at 50 km/h (half the speed), predict how much shorter the stopping distance would be — half as short, or something different? Justify your answer.
How close was your prediction?
Earlier you were asked: Why are car crashes at 100 km/h so much more destructive than at 50 km/h — even though the speed only doubled?
Now that you've worked through the lesson, write a fuller answer. Use the words kinetic energy, squared, and mass at least once each.
Q1. Explain why a car travelling at 100 km/h has four times the kinetic energy of the same car at 50 km/h, not twice as much. What does this mean for road safety? (3 marks)
Q2. A cricket ball and a soccer ball are bowled at the same speed. The cricket ball has more kinetic energy. Explain why, and describe one consequence of this for the sport. (3 marks)
Q3. Describe the energy transformations that occur when a hydroelectric power station generates electricity. Start from the water stored in a dam and end at a light bulb turning on. (4 marks)
Answers
▾MCQ 1
B — Kinetic energy depends on mass (how much matter the object has) and speed (how fast it is moving). Height relates to gravitational PE, not kinetic energy.
MCQ 2
C — The relationship between speed and kinetic energy is squared. Double the speed means 2² = 4 times the KE. This is why speeding is so much more dangerous than people expect.
MCQ 3
C — A 5 kg ball at rest has ZERO KE (no motion). The bullet is very light but extraordinarily fast — the speed effect (squared) dominates. Qualitatively, the high-speed bullet has far more KE than any of the slower options.
MCQ 4
C — When a car brakes, most kinetic energy is converted to thermal energy (heat) in the brake pads, discs and tyres. Some also becomes sound. None becomes gravitational PE (the car stays on flat ground).
MCQ 5
B — In a hydroelectric dam, flowing water carries kinetic energy. The turbines are spun by that kinetic energy, which is then converted into electrical energy by the generator.
Short Answer 1
Model answer: Kinetic energy depends on the square of the speed. When speed doubles (from 50 to 100 km/h), the KE increases by 2² = 4 times. This means a car at 100 km/h carries 4 times as much energy to be absorbed in a crash. For road safety, this means even small speed increases create dramatically larger impacts — which is why speed limits and speed cameras are placed precisely where they are.
Short Answer 2
Model answer: The cricket ball has more kinetic energy because it has greater mass than a soccer ball at the same speed, and KE increases proportionally with mass. In cricket, this means batters must react faster and more forcefully to a cricket ball than to a soccer ball moving at the same speed, and protective equipment (helmet, gloves, pads) must be stronger to absorb the higher KE impact.
Short Answer 3
Model answer: The water in the dam stores gravitational potential energy (GPE) due to its height. When released, it flows downhill — GPE converts to kinetic energy (KE) as the water speeds up. The fast-moving water hits turbine blades and spins them — KE of water converts to kinetic energy of the turbine. The spinning turbine turns a generator — KE converts to electrical energy. The electricity travels through transmission lines to the home. In the light bulb, electrical energy converts to light (and some heat). Each arrow in the chain is an energy transformation.