What causes friction in motion?

Friction in motion is primarily caused by resistance from surface contact between two objects. When two surfaces come into contact, the microscopic roughness and irregularities of those surfaces interact, leading to a resisting force that opposes the motion of one or both objects. This resistance is what we refer to as friction, and it plays a significant role in various everyday experiences, such as walking, driving, and the functioning of machines.

When a person walks, for instance, friction between their shoes and the ground allows them to push off and move forward. This resistance is necessary for controlling motion, stopping, and even starting movement. The amount of friction depends on factors such as the texture of the surfaces in contact, the weight of the objects, and whether the surfaces are stationary or sliding against each other.

While gravity, internal temperature changes, and materials can influence friction, they do not directly cause it. Gravity affects how much force is pressing the two surfaces together, which in turn can influence the amount of friction, but the actual cause of friction is the surface interaction itself.