A 1000 kg car at 20 m/s stops in 40 m. Estimate the braking force.
- Eₖ = ½ × 1000 × 20² = 200 000 J.
- F = 200 000 ÷ 40 = 5000 N.
Answer 5000 N
GCSE Combined Science revision · Physics: forces, waves and magnetism
The idea
When brakes are applied, work done by the friction force reduces the vehicle's kinetic energy, and the brakes heat up: Fd = ½mv². The faster the vehicle, the greater the braking force needed to stop it in a given distance. Large decelerations are dangerous: they can cause brakes to overheat and the vehicle to skid, and they injure passengers. Typical decelerations can be estimated from realistic masses and speeds.
- Find the kinetic energy: ½mv².
- Set work done by braking equal to it: F × d.
- Solve for the force or the distance.
Watch out Doubling the speed quadruples the kinetic energy, so for the same braking force the braking distance quadruples.
See it worked
Your turn
A 1200 kg car at 15 m/s is stopped by a braking force of 6000 N. What is the braking distance in metres?
If a car's speed doubles, its braking distance (same braking force):
Why are large decelerations dangerous?
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