Why Do Things Keep Moving?

Step 1 of 5

Everyone's instinct is wrong here.

For 2,000 years people thought moving things naturally slow down. Actually they only slow because something — air, friction, a wall — is stopping them.

Step 2 of 5

Remove the opposition and motion never ends.

A puck on smooth ice slides a long way. A probe in deep space, with nothing to rub on, keeps its speed for billions of years.

Step 3 of 5

More mass means more stubbornness.

Inertia grows with mass. Nudging an empty trolley is easy; nudging a loaded one takes far more effort to get moving and to stop.

Step 4 of 5

You feel it in every vehicle.

When a bus brakes you lurch forward — you were moving and you kept moving. When it accelerates you press back into the seat.

Step 5 of 5

Turning counts as changing too.

Inertia resists any change, including direction. In a fast corner you slide sideways because your body wants to keep going in a straight line.

This is exactly what a seatbelt does. In a crash the car stops; without a belt, you do not.