Newton’s laws of motion

Newton’s three laws explain how forces change motion. They explain why seatbelts matter, how rockets fly and why it’s harder to push a full trolley than an empty one.

Key facts

  • First law: with no resultant force, an object stays still or keeps moving at a steady velocity.
  • Second law: resultant force = mass × acceleration (F = ma).
  • Third law: when A pushes on B, B pushes back on A with an equal and opposite force.
  • Inertia: the tendency to keep doing what you’re doing; more mass, more inertia.

Worked examples

Example 1

A 1200 kg car has a driving force of 3000 N and resistive forces of 600 N. Find its acceleration.

  1. Resultant force: 3000 − 600 = 2400 N.
  2. a = 2400 ÷ 1200 = 2 m/s²

Answer: 2 m/s²

Example 2

Why does a gun recoil when it fires?

  1. The gun pushes the bullet forward.
  2. The bullet pushes the gun back with an equal force.

Answer: Newton’s third law