Newton's Laws of Motion, Explained

Why does a car's passengers lurch forward when it brakes suddenly? Why does a swimmer move forward by pushing water backward? Sir Isaac Newton answered questions like these with three laws that still describe how forces and motion connect.

First law — inertia

An object stays at rest, or keeps moving at a constant velocity, unless an unbalanced force acts on it.

This resistance to a change in motion is called inertia. It's why a seatbelt matters: when a car brakes, your body wants to keep moving forward at the same speed it was already travelling — the seatbelt is the unbalanced force that stops you from continuing in a straight line.

Second law — F = ma

The acceleration of an object depends on the force applied to it and its mass.

Force = mass × acceleration, or F = ma.

More force means more acceleration; more mass means less acceleration for the same force. Rearranging the formula lets you solve for whichever quantity you don't know.

Example: a 5 kg object is pushed with a force of 15 N. What's its acceleration?

a = F / m = 15 / 5 = 3 m/s²

Third law — action and reaction

For every action, there is an equal and opposite reaction.

A swimmer pushes water backward with their hands and feet; the water pushes the swimmer forward with exactly the same force in the opposite direction. The same idea explains how a rocket launches — it pushes exhaust gas downward, and the gas pushes the rocket upward.

In short

• First law: objects keep their state of motion unless an unbalanced force acts on them — this resistance is inertia.
• Second law: F = ma — force equals mass times acceleration.
• Third law: every action has an equal and opposite reaction.

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