Chapter 11, Problem 74A

(a)

To determine

To apply − the work energy theorem to describe the changes in the energy of the system when the car’s wheels do not skid.

Explanation of Solution

Introduction:

Work-energy theorem: According to work-energy theorem, the work done by the total forces exerting on a body is equivalent to the change in kinetic energy of it. That is,

Work done = Final kinetic energy− initial kinetic energy

Mathematically this can be represented as,

  $W=\Delta KE=K{E}_f-K{E}_i$

When the brakes applied but the wheels of the car do not skid, then there will be a rubbing between the brake surfaces against each other. This rubbing of brake surfaces works tostop the car. This work done by the brakes is equivalent to the difference in car’s kinetic energy. The loss in kinetic energy is transformed in to a heat energy and leads to heating of the brake surfaces.

(b)

To determine

To apply − the work energy theorem to describe the changes in the energy of the system when the brakes lock and the car’s wheels skid.

Explanation of Solution

When the brakes lock and the car’s wheels skid as the brakes applied by the driver, there will be a rubbing between the road and the wheels of the car. The work done by this process stops the car. In this situation, brakes will not be heated up instead surface of the tires gets heated up. This causes damage to the tire. Therefore, this is not an effectual practice to stop a car.