Question

What is the cause of friction.

Answer 1

Hint: When we think about the cause of friction, the most general idea that comes to our mind is the roughness of the surfaces in contact. However, roughness is not the only factor that causes the frictional force between surfaces. Think about some real life examples to get an idea about the factors.

Complete step-by-step answer:

Friction is a resistive force, which comes into play when there is a relative motion between two bodies in contact.
The frictional force between two bodies depends mainly on three factors: (i) the adhesion between body surfaces (ii) roughness of the surface (iii) deformation of bodies.
Let's discuss the three factors one by one.
(i) Adhesion: When two bodies come into contact, molecular forces attract the molecules of the both surfaces to each other. Since the surfaces of the two bodies attract each other at every point of contact, the bodies do not slide easily on each other. This adhesive property depends on the material.
You may have seen some water droplets sticking to the surface of the glass after drinking water. The frictional force between the water droplets and the surface of the glass prevent the water droplets from sliding down.
(ii) Roughness of the surfaces: When we think about friction, the first that may come to our minds will be roughness between bodies. It is easier to slide a body on a smooth surface whereas it is difficult to slide the same body on a rough surface.
(iii) Deformations: Soft materials will deform when under pressure. This also increased the resistance to motion. For example, when you stand on a rug, you sink in slightly, which causes resistance when you try to drag your feet along the rug’s surface. Another example is how rubber tires flatten out at the area on contact with the road.

Note: All the factors that affect friction between surfaces are material properties. They vary from material to material. We understand the degree of friction by a term called coefficient of friction $\mu $.
Sometimes, it is misunderstood that frictional force depends on the area of contact and friction is more if the area is more. However, this is not true. Friction is independent of the area in contact.