Question

Suppose your writing desk is tilted a little, a book kept on it starts sliding down. The frictional force acting on it is:
(A) upwards
(B) downwards
(C) no direction
(D) cannot say

Answer 1

Remember, frictional force is a resistive force i.e. a force which ‘opposes’ the motion of a body or tends to slow it down. This can only happen when it will act opposite to the ‘motion’.

Complete step by step answer:
As given, the book moves downwards. We know that frictional force always acts opposite to the direction of motion of the body. Hence, the frictional force acting on it is in upward direction.

Additional information:
Friction is a resistive force, which acts like resistance to the motion of one object moving relative to another. It always works in the direction in which the object is trying to move. It always slows down an object. It is of two types, Static friction and kinetic friction. The major factors which affect friction are mass of the object and the coefficient of friction. The heavier the mass of an object kept on a surface, the greater is the frictional force, and vice versa. The greater the value of coefficient of friction of a body, the greater will be the external force required to overcome its frictional force, and vice versa.

Note: Frictional force is provided by one body when it comes in contact with another. Even if an object is continuous in motion, the frictional force will continue to act, although it could be less or negligible. So, do not think that if the desk is tilted and the book is not stopping to slide, there is no frictional force.