Friction is a force that opposes the motion of objects that are touching and moving past each other. There are four main types of friction: static friction between non-moving objects, sliding friction between moving objects, rolling friction which is much less than static or sliding friction, and fluid friction caused by air or water resistance. Gravity is an attractive force that pulls objects towards the center of Earth. Falling objects experience both the force of gravity accelerating them downward and the resistance of air slowing them down, reaching a steady speed known as terminal velocity when these two forces balance.