This document discusses key concepts in kinesiology including inertia, friction, and levers. It defines inertia as the resistance of a body to changes in its motion or rest. Friction is defined as the force that opposes motion between two surfaces in contact. Levers are described as rigid bars that rotate around a fixed point called a fulcrum. There are three classes of levers defined by the relative positions of the fulcrum, effort, and weight. Mechanical advantage is gained when the effort arm is longer than the weight arm, allowing a smaller force to overcome a larger load.