Forces & Newton's laws

The core of mechanics: how forces change motion. From mass and inertia to Newton's three laws, built from everyday intuition all the way to the equations.

This is the heart of classical mechanics. It begins with mass and inertia, the reluctance of matter to change its motion, then defines speed, velocity and acceleration precisely so motion can be described exactly.

From there it builds Newton's three laws from intuition to formula: a body keeps its motion unless a net force acts (balanced forces), force equals mass times acceleration (F = ma), and every action has an equal and opposite reaction.

What you will learn:

  • Matter, mass and inertia; Newton's first law
  • Speed, velocity and acceleration
  • Newton's first, second and third laws, from words to equations
  • Mass versus weight, and the units of force

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Frequently asked questions

What are Newton's three laws in simple terms?
First: an object keeps its motion unless a net force acts on it. Second: force equals mass times acceleration (F = ma). Third: every force comes in an equal and opposite pair.
What is the difference between mass and weight?
Mass is how much matter an object contains and never changes. Weight is the force of gravity on that mass, so it changes with location, for example being lower on the Moon.