Inertia & Newton's laws

The bus brakes hard and all the passengers take a step forward at the same time. Nobody pushed them, yet they move. Behind this everyday moment are three famous laws that Isaac Newton wrote down in 1687 and that let you explain almost any motion: from a shopping trolley to a rocket.

Inertia: things stay as they are

The first law says: without a force, nothing changes. What is at rest stays at rest. What moves keeps moving straight ahead at the same speed. This stubbornness of things is called inertia. Without a force means, more precisely: in the end no force is left over. A book on the table is pulled down by gravity and pushed up just as hard by the table, so the two balance exactly and the book lies still. In the braking bus, by contrast, your body doesn't brake along, it wants to keep going. That's exactly why seat belts exist: they provide the force that slows you down together with the car.

F = m · a: force changes motion

The second law connects force, mass and acceleration: F = m · a. The unit of force is the newton, N for short, and 1 N accelerates 1 kg at 1 m/s². In plain terms: the more mass, the more force you need for the same acceleration. You can get an empty shopping trolley moving with one finger, but you have to lean into a full one. You see one special case every day: the Earth pulls every body downwards, and that force is called weight. You work it out with the same formula, using the acceleration of free fall, 9.8 m/s², as a: about 9.8 N acts on 1 kg.

Action and reaction

The third law says: forces always come in pairs. If you push against something, it pushes back just as hard. One point matters here: the two forces act on different bodies, which is why they don't cancel each other out. A rocket pushes gas backwards, and the gas pushes the rocket forwards. And when you push off the pool wall, you push the wall backwards and the wall pushes you forwards through the water.