In this IIT JEE physics video, we are going discuss one of the easiest topics, which is friction. In the video, you can find out what is friction, what is the cause of friction, factors affecting friction, and the relation of friction and relative motion between two surfaces. So what is the force of friction? Force of friction is nothing but the component of contact forces between surfaces which is parallel to surfaces in contact. Coming on to the factors affecting friction, the factors are:

1. Polishing reduces friction.
2. Lubrication reduces friction.
3. Smoothing the surface reduces friction.
4. Over-smoothing increases the friction

So how does over smoothing increase the friction? The molecules on a smooth surface are arranged in an order and the surface that is going to move over it is also arranged like crystals. So there will be a cohesive or adhesive force between the forces. These forces cause an increase in the interaction between the molecules of the surface. This results in the increase of the frictional force. Next topic discussed is the force of friction. Force of friction always opposes the relative motion between the two surfaces. Most of the times, students interpret this wrongly. You as a student should always remember that relative motion and motion are different. And you should also remember that the frictional force opposes the only relative motion. In the given video, you will find the answer why frictional force opposes relative motion only and not motion.