A force that opposes motion.

• A. Conduction
• B. Gravitational Force
• C. Normal Force
• D. Friction

Answer: (D)

Friction is the force that opposes motion between two surfaces in contact. It arises from the tiny bumps and irregularities on surfaces that catch on each other and from adhesive forces at the contact, which together resist sliding. When something moves, friction acts in the opposite direction of the motion, slowing it down; if the object is at rest, static friction prevents it from starting to move until a strong enough push is applied. The amount of friction depends on how hard the surfaces are pressed together (the normal force) and the materials involved, summarized by the idea that friction increases with normal force and varies with the surface textures. This is why a box is harder to push on a rough floor than on a smooth floor, and why ice is easy to slide on. Conduction is a way heat can transfer, gravitational force is the weight pulling downward, and the normal force is the support force from the surface acting perpendicular to it, not the force that resists motion along the surface.