Find the average frictional force needed to stop a car weighing 500 kg in a distance of 25 m if the initial speed is 72 km/h.
Initially, speed of the car kinetic energy of the car=72 km/h =72×5/18 m/s = 20 m/s
Kinetic energy at this speed = (1/2)×500×20×20 = 105 J
We need to bring down this kinetic energy to zero in order to stop the car.
The work done in this process = change in kinetic energy
= 0-105 = -105 J
Also, we know that work done = force × displacement
⇒ Force = work done/ displacement = -100000/25 = -4000 N
The negative sign indicates that the frictional force to be applied is in the direction opposite to that of displacement.
∴ Magnitude of frictional force required =4000N