Laws of dynamics

Dynamics studies the causes of changes in motion – the action of other bodies, i.e. force. Newton’s first law: there exist inertial reference frames in which a body stays at rest or in uniform straight-line motion when no other bodies act on it (or the actions balance). The force on a body and the acceleration it gets are proportional, and the size of the acceleration characterizes the body’s inertia – the inertial mass m: m = F/a. The second law: a = F/m, i.e. acceleration is directly proportional to the net force, inversely proportional to the mass, and directed along the force; the unit of force is 1 N = 1 kg·m/s². The third law: bodies act on each other with forces equal in size, along one line and in opposite directions: F₁₂ = –F₂₁. Because these forces act on different bodies they do not balance each other: each body gets an acceleration depending on its own mass. The parts of a rotating rigid body are not free, so the first law cannot be applied to them directly.