Name the force responsible for wearing out of bicycle tyres.

2 Does the use of ball bearing reduce or increase friction 3 What is the cause of friction? 4 Answer the following. (a) A ball is rolling on a road towards the north. What will be the direction of the frictional force acting on it?(b) If the direction of movement in the above case is changed to the south by applying force, then what will be the direction of the frictional force? 5 What is easier to move a box from rest, or to move it when it is already in motion?