Question

How can be friction counted under electromagnetic force? What is meant by the relative strength of the forces?

Answer 1

Friction is originated from electromagnetic forces and exchange forces between atoms and molecules. Electromagnetic forces and exchange forces (strong force) are two of the four fundamental forces, strong force, electromagnetic, weak force and gravity. The causes of the resistive force of friction are molecular adhesion (electromagnetic), surface roughness, and the plowing effect. Adhesion is the molecular force resulting when two materials are brought into close contact with each other. Surface roughness is a factor in friction when the materials are rough enough to cause serious abrasion. When one or more of the materials is relatively soft, much of the resistance to movement is caused by deformations or a plowing effect. So as you can see, electromagnetic forces are involved.
The relative strength of a force lets us decide for how long will that force act on a particle and the characteristic time tells us how long the force will need to act in order to affect the particle.