A car with a mass of 1000 kg travels at a speed of 18 km / h. After turning off the engine, the car drives 20 m and stops.

A car with a mass of 1000 kg travels at a speed of 18 km / h. After turning off the engine, the car drives 20 m and stops. What is the frictional force on the car? The friction force is considered constant.

After turning off the car engine, its accumulated kinetic energy will be completely converted into work of friction force.
Equality of kinetic energy and work of friction force:
(mv ^ 2) / 2 = FS, where v – vehicle speed, F – friction force, S – braking distance.
F = (mv ^ 2) / (2S).
Let’s substitute in the last formula the values from the condition, taking into account that 18 km / h = 18000 m / 3600 s = 5 m / s.
F = (1000 kg * 25 (m / s) ^ 2) / (2 * 20 m) = 625 (kg * m) / s ^ 2 = 625 N.
Answer: 625 N.