Consider pushing a heavy book across a table. As you push, you notice that you must exert a continuous force to keep the book moving. This resistance to motion is due to friction, a force that opposes the movement of objects sliding against each other. Friction can generate heat and slow down moving objects. The amount of friction depends on the surfaces in contact and the force pressing them together. Which statement best explains the role of friction in motion?
A) Friction is a force that speeds up objects as they move.
B) Friction opposes motion and can slow down or stop moving objects.
C) Friction has no effect on objects that are already in motion.
D) Friction only occurs when objects move through the air, not on solid surfaces.