Question

In order to stop a car in the shortest distance on a horizontal road, what should one do?
A. Apply the brakes very hard so that the wheels stop rotating
B. Apply the brakes hard enough to just prevent slipping
C. Pump the brakes (press and release)
D. Shut the engine off and not apply brakes.

Answer 1

Hint: Before we proceed with the problem, it is important to know about the frictional force. The frictional force is the force acting in the opposite direction of the motion of an object.

Complete step by step solution:
To stop a car over the shortest distance on a horizontal road, the maximum possible friction force by braking is required. The maximum force of friction can only be achieved when the surfaces are tightly in contact with each other and do not slip. In this situation, the limiting/maximum static force of friction comes into action.

When a vehicle is moving on a road, the wheel can be said to possess two velocities, one is its linear velocity due to the motion of the car in the forward direction, and the other is its angular velocity due to rotation about the axis of the wheel. When a car breaks suddenly, the rotation of the wheels stops reducing the angular momentum to zero.

But due to the momentum of the car the wheel slides forward on the road without rotating, leading the car to skid. When a car brakes suddenly just hard enough to prevent slipping, the angular velocity will decrease and friction will come into play to decrease the linear velocity and they will be in complete sync. So, to stop a car in the shortest distance, one should apply the brakes hard enough to just prevent slipping.

Hence, option B is the correct answer.

Note:If the brakes are applied only to stop wheels rotating, at a great speed, there is a chance of skidding or slipping. If you press and release the brakes, it will make the car move slowly, and stopping the engine will not help to stop the car for a long time because of the inertia of the car.