Summary
Dynamics studies why bodies move. Newton’s three laws sum it up: with no net force a body stays at rest or moves in a straight line at constant speed (inertia); a net force produces an acceleration, F = m · a; and if A pushes B, B pushes A with the same force in the opposite direction. Weight is a force, W = m · g, while mass does not change from one planet to another.
To solve a problem, draw every force on the body and add them as vectors: weight, the normal force, friction (F = μ · N) and tension. On an incline, weight splits into m · g · sin α, which pulls it down, and m · g · cos α, which the normal force balances. Momentum, p = m · v, is conserved when there are no external forces; turning needs a centripetal force, m · v² ÷ r; and two masses attract with F = G · m₁ · m₂ ÷ d².
Key points
- 1st law: no net force, rest or constant velocity.
- 2nd law: F = m · a (1 N = 1 kg · 1 m/s²).
- 3rd law: action and reaction, equal and on different bodies.
- Weight W = m · g (newtons); mass (kg) does not change from place to place.
- Friction: F = μ · N, always opposing motion.
- Incline: m · g · sin α down the slope; N = m · g · cos α.
- Momentum p = m · v; conserved without external forces.
- Centripetal force m · v² ÷ r; gravitation F = G · m₁ · m₂ ÷ d².