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 of classical mechanics.
- 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. Everyday slowing-down is really friction, itself a force.
- Second law — the net force equals mass times acceleration, . Acceleration is proportional to the net force and inversely proportional to the mass. This is the quantitative engine: given the forces, you get the acceleration, and (via calculus) the whole motion. It is a differential equation — — whose solution is the trajectory.
- Third law — for every action there is an equal and opposite reaction. Forces come in pairs acting on different objects: if A pushes B, then B pushes A with equal magnitude in the opposite direction.
The net force — the vector sum of all forces — is what governs the motion. 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. Common forces combine into the net: weight (), tension, friction, the normal force.
Because forces are vectors, they add by vector addition, and you often resolve them into components along chosen axes. Newton's second law then applies component by component — the bridge from the vector algebra of Unit 3 to physical prediction.
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 — force is required only to change velocity. And the third-law action-reaction pair acts on two different objects, so the two forces 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.