Which statement best describes friction?

• A. Friction is a force that speeds up motion.
• B. Friction is a force that opposes motion, often slowing objects down or changing direction.
• C. Friction only occurs in water.
• D. Friction has no effect on motion.

Answer: (B)

Friction is a force that opposes motion between two surfaces that are touching. It usually slows things down or makes them change direction because it acts opposite to the way the object is moving. For example, when you push a book across a desk, friction between the desk and the book slows it until it stops. Brakes work by creating friction to slow a bicycle, and the friction between tires and the road helps you turn and stop. Friction isn’t limited to water or to situations with moving through air; it happens on many surfaces and even when you’re standing still on the ground. It can be helpful, like giving you traction to walk without slipping, or it can slow things down and convert some of the moving energy into heat. The core idea is that friction always resists motion.