Why Things Move
Kinematics describes motion; dynamics explains it, through forces. A force is a push or pull, a vector with magnitude and direction. Newton's three laws are the foundation:
- First law (inertia) — an object at rest stays at rest, and an object in motion stays in uniform motion, unless acted on by a net force. Motion does not require a force to continue; only a change in motion does. This overturns the everyday intuition that things "naturally" slow down (which is really friction, a force).
- Second law — the net force equals mass times acceleration, . The acceleration is proportional to the net force and inversely proportional to the mass. This is the quantitative heart of mechanics: given the forces, you get the acceleration, and (via Unit 9's calculus) the whole motion.
- Third law — for every action there is an equal and opposite reaction. Forces come in pairs: if A pushes B, then B pushes A with equal magnitude in the opposite direction. Critically, these two forces act on different objects, so they do not cancel.
The net force — the vector sum of all forces — is what matters. An object in equilibrium (zero net force) has zero acceleration: it is at rest or moving at constant velocity. Only an unbalanced force accelerates an object. Weight (), tension, friction, and the normal force are common forces you combine into the net force.
Common pitfall: thinking a force is needed to keep an object moving, and misapplying the third law. By the first law, uniform motion needs no net force — a force is required only to change velocity. And the third-law action-reaction pair acts on two different objects, so they never cancel each other; forces that cancel to give equilibrium are different forces acting on the same object.
A block with two opposing force arrows; the accent net-force arrow (their vector sum) drives the acceleration a = F/m, with balanced arrows shown separately giving equilibrium.