Why things keep moving: inertia

You are standing on a bus. The driver suddenly hits the brakes. Whoosh! You lurch forward, even though nobody pushed you. What happened?

The bus puzzle

While the bus was moving, you were moving too, at the same speed. When the bus stopped, the bus stopped. But nothing stopped you. So your body kept going forward, just as it was before.

This is called inertia. Inertia is the tendency of things to keep doing exactly what they are already doing.

Newton's first law

More than 300 years ago, Isaac Newton wrote down this idea as the first of his three laws of motion:

A thing that is still stays still, and a thing that is moving keeps moving in a straight line at the same speed, unless a force acts on it.

There are two halves:

• Still things stay still. A football sits on the grass until somebody kicks it.
• Moving things keep moving. A ball keeps going in a straight line at the same speed, until something pushes on it.

But things on Earth always stop!

This is the part that fools almost everybody. When you roll a ball along the floor, it stops. So it seems that moving things naturally stop.

They do not. The ball stops because of friction and air resistance, which are forces that push against it. If you could get rid of them, the ball would keep on rolling forever.

Ice shows this well. A puck on ice slides a very long way, because friction is tiny.

Space is the best place to see inertia

In space there is almost no air and no friction. A spacecraft that has switched off its engines keeps coasting at the same speed, in a straight line, for years. It does not need fuel to keep moving. It only needs fuel to change how it moves.

In Orbit Keeper, the satellite is launched sideways and it just keeps going sideways. Gravity is the only force bending its path. Without gravity, it would fly off in a straight line, just as Newton's first law says.

Seat belts and headrests

Inertia explains lots of safety rules:

• In a car crash, the car stops suddenly, but your body wants to carry on forward. A seat belt is the force that stops you.
• A headrest stops your head from snapping back when the car is hit from behind.
• It is why you must never stand on a moving bike, bus or scooter without holding on.

More mass, more inertia

Is it easier to stop a rolling tennis ball or a rolling bowling ball? The bowling ball, of course. Things with more mass have more inertia. They are harder to start, harder to stop and harder to turn. We will use this idea in the next lesson.