Newton's Three Laws of Motion

The foundation of classical mechanics

In 1687, Isaac Newton published three laws that describe the relationship between an object and the forces acting on it. More than three centuries later, these laws still explain almost everything we see move — cars, planets, athletes, and spacecraft. Together they form the heart of classical mechanics.

A force is a push or a pull, measured in newtons (N). The key insight of all three laws is this: forces do not cause motion itself — they cause changes in motion.

First Law: the law of inertia

An object at rest stays at rest, and an object in motion continues at constant velocity in a straight line, unless acted on by a net external force.

This property — the resistance of an object to a change in its motion — is called inertia, and it increases with mass. A loaded truck is far harder to start or stop than a bicycle.

Real examples:

• A puck slides almost forever on smooth ice because friction (the force that normally stops it) is tiny.
• When a car brakes hard, your body lurches forward. Nothing pushed you — your inertia simply kept you moving until the seatbelt applied a force.

The "net" part matters: many forces can act at once, but only an unbalanced total force changes motion.

Second Law: force, mass, and acceleration

The Second Law makes the First Law quantitative. The acceleration of an object depends on the net force on it and its mass:

F = m × a

where F is net force (N), m is mass (kg), and a is acceleration (m/s²). Force and acceleration are vectors — they point in the same direction.

Two consequences fall straight out of this:

• For a fixed mass, more force means more acceleration.
• For a fixed force, more mass means less acceleration.

Worked example: A net force of 12 N acts on a 3 kg trolley.

a = F / m = 12 N ÷ 3 kg = 4 m/s²

This is why an empty shopping trolley darts forward with a light push, while a full one barely moves under the same push — its larger mass demands more force for the same acceleration.

Third Law: action and reaction

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

Forces always come in pairs. If object A pushes on object B, then B pushes back on A with a force that is equal in size but opposite in direction. Crucially, the two forces act on different objects, which is why they do not simply cancel out.

Real examples:

• Walking: your foot pushes backward on the ground; the ground pushes you forward.
• Rockets: the engine throws hot gas downward; the gas pushes the rocket upward. This even works in the vacuum of space, where there is nothing to "push against" — the reaction force comes from the expelled gas itself.
• Swimming: you push water backward with your arms; the water pushes you forward.

Putting the laws together

Imagine kicking a football:

1. First Law — the ball sits still on the grass because the forces on it are balanced.
2. Second Law — your kick supplies a net force; the lighter the ball and the harder the kick, the greater its acceleration (F = ma).
3. Third Law — your foot pushes the ball, and the ball pushes back on your foot with equal force (you feel the impact).

To explore where the energy of that kick goes, see the many forms of energy. To see how forces are made easier to apply, read about simple machines.

Try it yourself! 🧪

A balloon rocket demonstrates all three laws.

1. Thread a long string through a drinking straw and stretch it tightly between two chairs.
2. Inflate a balloon, pinch the neck shut, and tape it to the straw.
3. Let go. The balloon shoots along the string.

What just happened? At rest the balloon obeyed the First Law. Releasing the air created a net force (Second Law) — and the air rushing backward pushed the balloon forward (Third Law). Try a bigger balloon or a heavier load taped on, and use F = ma to predict whether it accelerates faster or slower.