Define friction. What types are there?

• A. Friction is a magnetic attraction between materials.
• B. Friction is a resistive force between surfaces; static and kinetic friction.
• C. Friction is the force that accelerates an object.
• D. Friction is a push initiated by gravity on an inclined plane.

Answer: (B)

Friction is a resistive force that acts at the contact between surfaces in contact, opposing their relative motion or the attempt to move.

The two main types are static friction, which prevents motion when objects are at rest relative to each other, and kinetic (sliding) friction, which acts once motion has started and opposes that motion. Static friction can increase up to a maximum value to keep things from moving, while once motion begins, kinetic friction typically acts at a lower, roughly constant level as surfaces slide past one another. Friction is not a magnetic attraction, nor is it the force that makes things accelerate; it’s the resisting force that slows things down or keeps them from starting to move, and its size depends on the roughness of the surfaces and the normal force between them.