Friction opposes the motion of objects and causes them to slow down and stop when in contact with a surface. There are different types of friction including rolling, sliding, fluid, and static friction. Rolling friction occurs when an object rolls over a surface. Sliding friction, also called dry friction, happens when two objects rub or slide against each other. Fluid friction is the drag between moving objects and liquids or air. Static friction prevents stationary objects from starting to move when resting against a surface.