The force of friction occurs primarily because: A) two surfaces in contact have magnetic forces of attraction.

B) on the microscopic level, two surfaces in contact are rough even if they appear smooth to the touch.

C) two surfaces in contact have a gravitational attraction to one another.

D) both A and B.

B

Explanation:

Friction is a force that opposes motion between any surfaces that are touching. Friction occurs because no surface is perfectly smooth Friction produces heat because it causes the molecules on rubbing surfaces to move faster and have more energy.

Sound travels more quickly through solids than through liquids and gases because the molecules of a solid are closer together and, therefore, can transmit the vibrations (energy) faster. Sound travels most slowly through gases because the molecules of a gas are farthest apart.