Friction is a force that opposes motion between surfaces that touch. It acts along the surface, always against the motion or the attempted motion. Static friction holds a resting object still and is usually larger than the kinetic friction on a moving one. Air resistance, which scientists call drag, is a force from the air on anything moving through it. Drag grows with speed and with the size and shape of the object facing the air. A ball rolls farther on a smooth floor than on carpet because the floor pushes back less, and a crumpled sheet of paper falls faster than a flat one because the air pushes back less.