Question

Automobile tyres have generally irregular projections over their surfaces. Why?

Answer 1

Irregular projections are made on the surface of tyres to increase friction.
Friction is a force which acts parallel to the surface of the bodies which are in contact with each other. It opposes the relative motion between the two surfaces.

Complete step by step answer:
According to Newton’s first law of motion, “Every body continues to be in its state of rest or motion in a straight line unless compelled by some external force to act otherwise.”
Let us take an example of the motion of a car starting from the rest. When the car is stationary, no net force is acting on it. But when it accelerates, according to the first law of motion, it will require a net external force to start moving. Therefore, acceleration of a car is not possible due to any internal source. But the only possible external force present is friction. We can conclude that it is the frictional force which accelerates the car. If friction will not be present, the car cannot accelerate by itself.
Let us see how. When we accelerate the car in forward direction, the tyres of the car move clockwise. The surface of tyre which is in contact with the road moves backward. The frictional force opposes this motion and hence the car moves in forward direction.
This is why automobile tyres have irregular projections over their surface.

Note: Friction force not only helps the automobile to accelerate during pickup. It also plays an important role when the automobile takes a turn, and the vehicle undergoes circular motion. Friction also provides the centripetal force required to turn around and prevent the vehicle from skidding.