1. Consider a block placed on a horizontal floor and give
it an initial push (figure a). The block will stop after
travelling some distance. According to Newton's second
law, a retarding force must be acting on the block. This
opposing force is called the friction force. The friction
force always acts along tangential direction at the point
of contact and in a direction opposite to the direction of
relative motion between the two surfaces.

2. Now consider a block placed at rest in an accelerating
cart as shown in figure (b). The block in fact accelerating
along with the cart. Which force causes the acceleration
of the block ? It is clear that the only force in the
horizontal direction is the frictional force. If there were no
friction, the surface of the cart would slip and the block
would remain at its initial position by inertia. Thus we can
say that sometimes friction is required to start the
motion. Its direction may be backward or forward of the
motion of the body. Friction can be defined as a force
which opposes the relative motion between the surfaces
in contact.


Friction can be defined as a force which opposes the
relative motion between the surfaces in contact.