In physics, the primary force that stops or slows down the motion of an object is friction. Friction is the resistance that one surface or object encounters when moving over another. There are several types: dry friction (resisting the motion of solid surfaces), fluid friction (resistance through liquids or gases, often called drag or air resistance), and internal friction (resistance within the material itself). When an object moves, friction acts in the opposite direction of the motion, converting kinetic energy into heat. Another force that can stop an object is normal force or applied force in the opposite direction, such as a wall stopping a moving ball. On a larger scale, gravity can also act as a slowing force if an object is moving upward. However, in most everyday contexts—like a car braking or a sliding book coming to a halt—friction is the retarding force responsible for the deceleration. Without friction, an object in motion would theoretically stay in motion indefinitely unless acted upon by another external force.