The Laws of Motion
Steady motion needs no force at all; forces only explain changes in motion. The second law runs both ways: ask what net force acts to find acceleration, or see acceleration to know the net force.
Newton's three laws form classical mechanics:
| Law | Core Rule | Statement |
|---|---|---|
| First (Inertia) | Zero Net Force | A body stays at rest or in uniform motion unless acted on by a net external force. |
| Second | Only the vector sum of all forces determines acceleration. | |
| Third | Action-Reaction | If body A exerts a force on body B, B exerts an equal and opposite force on A. |
Common pitfall: Action-reaction pairs never cancel because they act on different bodies. The book's weight and the table's normal force on it are NOT a third-law pair; both act on the book itself.
Free-Body Diagrams & Forces
Free-body diagrams are the essential tool for applying Newton's laws: isolate the object, draw all forces acting on it, choose coordinate axes, and apply and .
Common forces quantify these interactions:
| Force | Formula | Direction | Meaning |
|---|---|---|---|
| Weight | Downward | Gravitational pull ( is mass, is gravity). | |
| Normal | Perpendicular | Support force from a surface. | |
| Kinetic friction | Opposing motion | Friction during sliding ( is kinetic coefficient). | |
| Static friction | Opposing tendency | Friction before sliding starts ( is static coefficient). | |
| Tension | Along string | Pulling force exerted by a taut rope or string. |