What is a contact force that objects exert on each other when they rub together.

Frictional force is an opposing force which acts when two bodies in contact tend to move relative to each other. Frictional force is a contact force which acts when two surfaces rub against each other. It depends on the roughness of the surface and area of contact.

The roughness is due to microscopic hills and valleys characteristic. When two surfaces move with respect to one another, the hills and valleys interlock and oppose the motion.

The force of friction always acts in opposite direction of motion. static friction acts when body tends to move. In motion, Kinetic friction acts. Static friction is always greater than kinetic friction.

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