Movement energy
Why does a fast bowling ball knock pins over much harder than a slow one?
Play
Explain
Moving things have energy.
Fast things have lots of it.
Anything moving has kinetic energy, or movement energy.
A heavier object has more kinetic energy, and a faster one has much more.
Speed matters most. If you go twice as fast, you have four times the energy.
KE = ½mv². Because speed is squared, it has a bigger effect than mass.
Double the mass and KE doubles. Double the speed and KE becomes four times bigger.
Worked example: a 50 kg child at 4 m/s has KE = ½ × 50 × 4² = 400 J.
- Double the mass: double energy
- Double the speed: 4 × energy
- Energy does the work
Want the full story?
Close to the speed of light, this formula needs correcting. There is also rotational kinetic energy, so a rolling ball has some energy from spinning too.
The Formula
movement energy = half × mass × speed × speed
- the kinetic energy, measured in joules (J)
- the mass, measured in kilograms (kg)
- the speed, measured in metres per second (m/s)
Try the numbers
Kinetic energy: 625 J
Try it at home
You need a ball, some light plastic cups or card tubes.
You need: a ball, some light plastic cups or card tubes.
Roll a ball at some cups. Crash!
- Stand some cups in a group.
- Roll a ball gently into them.
- Reset, then roll it faster.
What's happening: the faster ball has much more kinetic energy, so it knocks the cups much further.
Physics is life
Your body
Running uses movement energy.
Daily life
Fast cars need more room to stop.
Nature
Wind has movement energy.
Space
Meteors move incredibly fast.
Quick check
Question 1 of 3
Words to know
Kinetic energy: The energy something has because it is moving.
- Kinetic energy
- The energy something has because it is moving.
- Joule (J)
- The unit of energy. It takes about 1 joule to lift an apple 1 metre.
- Speed
- How far something goes in each second.