Friction is

Friction is

• A. A force which opposes the relative motion between two objects
• B. A force that always acts in the same direction as motion
• C. Always increases energy
• D. Occurs only in vacuum

Answer: (A)

Friction is the force that opposes the motion of one object sliding over another (or the tendency to slide). It arises at the interface of contacting surfaces because of microscopic roughness and interactions between their molecules. There are two practical forms: static friction, which resists the start of motion up to a maximum, and kinetic (sliding) friction, which acts while there is relative motion. In all cases, the friction force points opposite to the direction of the relative motion (or the impending motion) between the surfaces, converting some of the mechanical energy into heat. This is why friction isn’t limited to vacuum and isn’t always in the same direction as motion, and why it doesn’t add energy to the system. The key idea is that friction always resists the relative motion of the touching surfaces.

Friction is the force that opposes the motion of one object sliding over another (or the tendency to slide). It arises at the interface of contacting surfaces because of microscopic roughness and interactions between their molecules. There are two practical forms: static friction, which resists the start of motion up to a maximum, and kinetic (sliding) friction, which acts while there is relative motion. In all cases, the friction force points opposite to the direction of the relative motion (or the impending motion) between the surfaces, converting some of the mechanical energy into heat. This is why friction isn’t limited to vacuum and isn’t always in the same direction as motion, and why it doesn’t add energy to the system. The key idea is that friction always resists the relative motion of the touching surfaces.