Slide a book across a table and it slows down. The force that stops it is friction. Friction is a force that resists motion between surfaces in contact. It arises from the microscopic roughness of surfaces and the intermolecular forces between them. Even surfaces that look smooth have tiny bumps and valleys that catch on each other. Friction converts kinetic energy into heat, which is why rubbing your hands together warms them up.

There are two main types of friction. Static friction acts on objects at rest. It prevents them from starting to move. Kinetic friction acts on objects in motion. It slows them down. Static friction is usually stronger than kinetic friction, which is why it takes more force to start a box moving than to keep it moving. Friction depends on the nature of the surfaces and the normal force pressing them together. It does not depend on the area of contact, which is counterintuitive but experimentally confirmed. Friction is essential in everyday life. Without it, you could not walk, drive, or hold a pencil. But it also wastes energy and wears out machines.

Friction facts

• Definition. Force resisting motion between surfaces.
• Static. Acts on objects at rest.
• Kinetic. Acts on objects in motion.
• Depends on. Surfaces and normal force.
• Independent of. Area of contact.

Friction is not the same as drag. Friction acts between solid surfaces. Drag acts between a solid and a fluid.