Why Things Move
Kinematics describes motion; dynamics explains it using forces (pushes or pulls, which are vectors). Newton's three laws form the foundation of mechanics:
| Law | Definition | Core Meaning |
|---|---|---|
| First law | Object stays at rest or in uniform motion unless acted on by a net force | Motion needs no force to continue; only a change in motion does |
| Second law | (Net force equals mass times acceleration) | Acceleration is proportional to net force and inversely to mass |
| Third law | For every action, an equal and opposite reaction | Forces come in pairs acting on different objects; they never cancel |
The net force is the vector sum of all forces (, tension, friction, normal force). An object in equilibrium has zero net force and zero acceleration: it is at rest or moving at constant velocity.
Forces in Action
The quantitative heart of mechanics is . Given the forces, you find the acceleration, and thereby the full motion.
Consider a block driven by an unbalanced net force. The resulting acceleration is dictated directly by , while balanced opposing forces result in equilibrium.
Common Pitfalls: - Thinking a force is needed to keep an object moving. Uniform motion needs no net force; a force is only required to change velocity (overturning the everyday illusion of friction). - Misapplying the third law. Action-reaction pairs act on two different objects, so they never cancel. Forces that cancel to create equilibrium are different forces acting on the same object.