Speed Counts Twice
- Physics
- Ages 11–14 · Grades 6–8
- Hands-on experiment
What you’ll find out
- Kinetic energy depends on mass and on speed multiplied by itself.
- Doubling the speed gives four times the energy, not twice.
- Braking distance grows with energy, which is why speed limits matter.
No sign-up and no real names. In the lesson you check your understanding as you read, run the experiment yourself, and practise until it sticks.
Doubling the speed does not double the energy
A moving object has energy, called kinetic energy. Two things decide how much. The first is mass: double the mass and you double the energy. The second is speed, and speed does not work the same way. Doubling the speed gives four times the energy, and tripling it gives nine times. Take a car with a mass of 1,000 kg. At 10 metres per second it carries 50,000 joules. At 20 metres per second it carries 200,000 joules, and at 30 it carries 450,000.
Why stopping distances grow so fast
Brakes work by turning kinetic energy into heat in the brake discs, and they can only do that at a certain rate. So a car with four times the energy needs about four times the distance to stop. You also have to add the thinking distance, which is how far the car travels while the driver reacts. Put them together and the total grows quickly. At 20 miles per hour a car needs about 12 metres to stop, but at 40 miles per hour it needs about 36 metres. Twice the speed, three times the distance.
Curriculum links: NGSS MS-PS3-1