Friction is the force that opposes relative motion, or the tendency of motion, between two surfaces.
It acts parallel to the surfaces in contact and generally points opposite the direction in which one surface would slide relative to the other. Friction arises from microscopic surface irregularities, deformation, and interactions between materials. It converts some mechanical energy into heat and can also cause wear.
Static friction prevents motion from beginning, such as when a box remains still while you push it gently. Kinetic friction acts after surfaces begin sliding. In many material pairs, the maximum static friction is greater than kinetic friction, which explains why starting a heavy object moving can require more force than keeping it moving.
Friction is not always an unwanted force. It lets shoes grip the ground, allows car tires to accelerate and brake, and makes writing with a pencil possible. Rolling friction and fluid friction are related forms, while lubrication reduces friction between moving machine parts. The coefficient of friction depends on the materials and their surface conditions.