Factors affecting friction

Question 1 On what factors does the friction depends?

Question 2 Which will cause more friction: a rough surface or smooth surface?

Question 3 Ramesh has to push a lighter box and Seema has to push a similar heavy box on the same floor. Who will have to apply a larger force and why?

Question 4 Give an example to show that friction depends on the nature of two surfaces?

Question 5 Give an example to show that friction depends on the force with which the two surfaces are pressed together?

The Friction between two Surfaces depends on two Factors

(1) The nature of the two surfaces i.e. smoothness or roughness of two surfaces

(2) The force with which two surfaces are pressed together.

(1) Dependence of Friction on the Nature of two Surfaces

Friction depends on the smoothness or roughness of the two surfaces which are in contact with each other.

(a) When the two surfaces are in contact are smooth, then the friction between them will be small because the interlocking of smooth surfaces is less. As the degree of roughness of the two surfaces in contact increases, the friction also increases.

(b) When the two surfaces in contact are very rough, then the friction between them will be very large because the interlocking of very rough surface  is too much.

Activity

Place a brick on the floor. Tie a string the around the brick and connect it to the hook of a spring balance. Apply a  pulling force to the brick by pulling the other end of spring balance by hand till the brick just begin to slide on the floor. Note down the reading on spring balance. This reading of spring balance will give us the magnitude of force of friction between the surface of floor and the surface of brick.

(a) Now wrap a piece of polythene around the brick and repeat the above activity. We note the spring balance reading when the polythene wrapped brick just begin to slide on the floor. This reading of spring balance is smaller than the first reading of spring balance indicating that the force of friction has decreased. Wrapping of polythene sheet makes the surface of bricks smooth due to which the friction with floor decreases.

(b) Now wrap a jute cloth around the brick and repeat the above activity once again. Note the spring balance reading when the jute Wrapped brick just begins to slide on the floor. The reading of spring balance is greater than the first reading of spring balance indicating that the force of friction has increased. Wrapping of jute cloth makes a surface of brick more rough due to which the friction with the floor increases.

(2) Dependence of Friction on the force with which two Surfaces are pressed together

Friction is caused by the interlocking of irregularities of the two surfaces when one object is placed over another object. If the two surfaces of objects are pressed together harder by a great force, then the friction will increase.

Activity

Suppose we have two boxes of same size but one box is light and other box is heavy. If we push both the boxes on the floor ,one by one, we will find that we have to apply only a small force to make the lighter box move on the floor but a much larger force has to be applied to make the heavier box move on the floor. This shows that there is less friction between the light box and floor but much more friction between the heavy box and the floor. The weight of a box is also a force which acts in the downward direction. Because of its smaller weight, the light box presses on the floor with less force and hence the friction between lighter box and floor is less. This lesser force of friction allows the lighter box to be moved easily by applying smaller push.

Because of its greater weight the heavy box presses on the floor with greater force. Since the surface of heavy box and floor are pressed together harder the friction between them increases and become much greater. This greater friction does not allow the heavy box to be moved on the floor by applying a small force. A much larger force of our Push has to be applied to make the heavy box move on the floor.