Bob is in his car that has a kinetic energy of 450000 J. When bob slams on his brakes and skids to a stop in 20 meters, what is the average friction force acting on the car to bring it to stop?

Answer 1

As a first approximation, the force of friction is doing the work of 450 KJ, to bring the car to a stop (by the Work-Energy theorem). Since the distance over which the car skids is 20 m, the average force of friction is

450,000 / 20 = 22,500 N

which is pretty unrealistic. A skidding car has a deceleration of about 0.5 g, or approximately 5 m/s², so that this car would have a mass of about 4,500 kg (about 10,000 lb): a bit too heavy for a car, even when Bob is in it.

(I said as a first approximation because some of the friction is occurring within the car, in the braking system itself (unless Bob locked up the wheels as soon as he slammed the brakes) and so would not be acting on the car. Though, judging by the unrealistic figure, it is unlikely that the problem is concerned with such subtleties.)