Laws of Motion

No matter how quiet or calm it might seem, nothing in our world is ever still. Deep inside every object, even in the air that surrounds us, atoms and molecules are restlessly moving.

All motion - even the random dance of atoms - is caused by forces pushing and pulling. Forces act logically, so most things in the world move in ways we can understand and predict. For this, we use the three laws of motion: a simple set of scientific rules first thought up over 300 years ago.

First law of motion

All things will either stay still or move with a steady speed unless a force acts on them. This idea is called inertia. The more mass something has, the more inertia it has too - so it will be more likely to stay still or resist changes to its motion.

At rest: A soccer ball rests on the ground because there is no overall force acting on it. Its weight is perfectly balanced by the ground, and there is no sideways force.

Force applied: When you kick the soccer ball, you apply a force to one side. There’s nothing to counteract or balance this force so it makes the ball move.

Motion stopped: The ball shoots through the air or rolls across the ground with a fairly steady speed until it hits something, such as your foot. When your foot applies a force, the ball stops moving.

Second law of motion

When a force acts on something, it makes it accelerate (go faster, slower, or change direction). The amount of acceleration depends on the size of the force and the mass of the object. The bigger the force or the lighter the object, the more it accelerates.

Force = Mass x Acceleration

Acceleration is how fast speed (meters per second) is changing over time (per second). It is measured in meters per second per second (ms-2).

Small mass, small force: Kick a ball lightly (with a small force) and it accelerates at a steady rate. The acceleration is equal to the size of the force divided by the mass of the ball.

Small mass, medium force: Kick the same ball with twice the force and you get twice the acceleration. If it flies in a straight line, it accelerates to twice the speed in the same time.

Double mass, large force: A heavier ball needs a bigger force to get it moving. If you use eight times your original force and the ball is twice as heavy, you get four times the original acceleration.

Third law of motion

Newton’s third law says that when a force acts on something, there is always another force just as big acting in the opposite direction. If the original force is called the action, the opposite force is called the reaction. That's why the third law is often written in a shortened form: every action has an equal and opposite reaction.

Action: Imagine you are on a skateboard and you push against a wall (action). The wall pushes back with a reaction force, causing you to roll away from the wall.

Reaction: If you push a friend, who is also on a skateboard, your force (action) moves them away from you. The reaction force makes you move in the opposite direction at the same speed as your friend.