Find the average

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