Newton's Three Laws of Motion
Why you lurch forward when a bus brakes, why it's harder to push a full trolley, and how rockets fly. Three rules from 1687 explain them all.
~8 min3 lessonsF = ma calculator5-question quiz
01
First law: things keep doing what they're doing
An object stays still, or keeps moving at the same speed in a straight line, unless a force acts on it. This tendency is called inertia.
- When a bus brakes, your body keeps moving forward. That's why seatbelts matter.
- A puck on ice glides a long way because there's little friction to slow it.
02
Second law: force = mass × acceleration
F = m × aforce in newtons (N) · mass in kilograms (kg) · acceleration in metres per second squared (m/s²)
The heavier something is, the more force it takes to speed it up by the same amount. Push a full shopping trolley and an empty one equally hard: the empty one speeds up much faster.
Example · a 1,000 kg car accelerating at 3 m/s²
- F = 1,000 × 3
- 3,000 N of force
Try it · F = ma calculator
03
Third law: every push has a push back
For every action there's an equal and opposite reaction. Forces always come in pairs.
- A rocket pushes hot gas down; the gas pushes the rocket up.
- When you walk, your foot pushes the ground backwards; the ground pushes you forwards.
- Jump off a small boat and it drifts backwards as you go forwards.
Why doesn't everything cancel out? The two forces act on different objects. The ground isn't pushed by its own force; you are.