What is friction?

A. the opposite of inertia

B. the opposing force of gravity

C. an opposing force between two objects

D. a force that keeps objects moving in a straight line

The best answer is C.

The force that opposes the motion of an object is called friction.

There are two kind of friction namely kinetic and static friction forces.

Kinetic friction is the force that is exerted on one surface by another when the two surfaces rub against each other because one or both surfaces are moving.

Static friction is the opposing force that acts against a secondary force that is trying to cause a stationery object to start moving. If there is no such force acting on the subject, the static friction is nil or zero.