Physics
Newton's Laws
Three Laws
The foundation of classical mechanics is Newton's three laws.
- First law (inertia): if there is no external force, or if the forces are balanced, a body stays at rest or moves in a straight line at constant speed.
- Second law: a = F/m (or F = ma). Acceleration is directly proportional to the force and inversely proportional to the mass.
- Third law: F₁₂ = −F₂₁. The action and reaction forces are applied to different bodies, so they do not cancel each other out.
Forces
| Force | Formula |
|---|---|
| Weight | F = mg (g ≈ 9,8 ≈ 10 m/s²) |
| Friction | F = μN |
| Elastic force (Hooke) | F = kx |
| Universal gravitation | F = G·m₁m₂/r² |
Example: if a force of 30 N acts on a 6 kg body, a = 30/6 = 5 m/s². The weight of a body of mass 4 kg is F = 4·10 = 40 N.
Net Force
Forces acting in opposite directions along one straight line are subtracted, and those acting in the same direction are added. You find the acceleration from the net force: a = F_net/m.
- Draw all the forces acting on the body
- Find the net force F
- Calculate a = F/m
On a horizontal road, the sliding friction force equals F = μ·m·g, because if there is no other vertical force, the normal force is N = mg.