What Causes Friction?

Answer: 

Friction is the resistance against the relative motion between solid surfaces. We might have seen smooth surfaces a lot. But, having a surface completely smooth and without any unevenness or roughness is next to impossible.

Since every surface has at least a minimum amount of roughness, friction force takes its value.

However, friction has another major cause, which is Normal force applied on both or any of the correlated surfaces. As the normal force changes, the friction takes different values.

To attain zero friction, an ideally smooth surface or zero normal force will be required. However, the value of friction is derived from the following formula:

f = μ N

f = friction force,

μ = friction coefficient of a particular surface,

N = Normal or perpendicular force.