Answer :

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


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