Newton's Three Laws
Published by Isaac Newton in 1687 in the Principia Mathematica, they are the basis of classical mechanics.
1st Law — Inertia
"Every body remains at rest or in uniform straight-line motion unless a net force acts on it."
2nd Law — Fundamental Principle
F = m · a
The net force on a body equals the product of its mass and its acceleration. Unit: 1 N = 1 kg·m/s².
3rd Law — Action and Reaction
"For every action there is an equal and opposite reaction." Action–reaction pairs act on different bodies — that is why they never cancel out.
Inclined plane
On a plane with angle θ, the weight (P = m·g) breaks down into:
- Px = m·g·sin θ — pulls the body down the plane
- N = m·g·cos θ — presses against the surface
- Ff = µ · N — friction (opposite to the motion)
FAQ
What is the difference between mass and weight?
Mass (kg) is the amount of matter, constant everywhere. Weight (N) is the gravitational force acting on that mass: P = m·g. On the Moon your mass is the same but your weight is about 6 times smaller.
What does "net force" mean?
It is the vector sum of all applied forces. For a body in equilibrium, F_net = 0. For accelerated motion, F_net = m·a.
Why does friction appear as µ·N and not µ·P?
Because friction is proportional to the force pressing the surfaces together — the Normal force, not the total Weight. On a horizontal plane N = P; on inclines, N = P·cos θ < P.
When should I use g = 9.8 or g = 10?
In many exams g = 10 m/s² is used to make the arithmetic easier. In real measurements and more rigorous exercises, g = 9.8 m/s² (average value at sea level).