When two surfaces experience friction with each other, the particles on the surfaces are affected in two ways: movement and spacing. The particles on the surfaces move against each other, creating a force that resists the motion of the two surfaces. This force is known as friction. At the same time, the particles on the surfaces are also pushed apart, creating a small gap between the two surfaces. This gap is known as the coefficient of friction, and it affects the amount of friction that is experienced between the two surfaces.
