Skip to content

The first law

Forces and motion · level 5, lesson 1 of 6

A question to start

A spacecraft on its way to Mars fires its engine for a few minutes after launch, then switches it off. It still has hundreds of millions of kilometres to go, and seven months to do it. What keeps it moving?

The idea

Newton's first law: an object stays at rest, or keeps moving at a steady speed in a straight line, unless an overall (net) force acts on it.

So nothing is needed to keep something moving. A force is needed only to change its velocity: to start it, stop it, speed it up, slow it down or turn it. On Earth, things seem to need a push to keep going only because friction and air keep slowing them down.

This tendency to keep doing what it is doing is called inertia.

engine off: it keeps going

Between the planets there is almost no air and nothing to rub against. A spacecraft to Mars coasts most of the way. Only the Sun's gravity bends its path into a long curve. Its engines fire for a few small course corrections.

Worked examples

1. Seat belts. A car stops suddenly. Your body has inertia: it keeps moving forward at the car's old speed until something stops it. The seat belt provides that force; without it, the dashboard or the windscreen would.

2. The tablecloth trick. Yank a tablecloth out fast, and the plates on it hardly move. The friction acts for too short a time to change their velocity much.

3. Voyager 1. Launched in 1977, it is still moving out of the solar system at about 17 km/s. Since its last planet flyby, Saturn in 1980, it has coasted on inertia, firing only small thrusters to keep its antenna pointed at Earth.

Try it

  1. State Newton's first law in your own words.
  2. A hockey puck slides on ice at a nearly steady speed. What does that tell you about the forces on it?
  3. Why do passengers lurch forward when a bus brakes?
  4. A spacecraft between Earth and Mars has its engines off. Is it slowing down because nothing is pushing it?
Answers
  1. Without an overall force, a thing at rest stays at rest, and a moving thing keeps the same speed and direction.
  2. They nearly balance: almost no overall force.
  3. Their bodies keep moving forward at the old speed while the bus slows under them.
  4. No. It needs no push to keep moving. Only the Sun's gravity changes its speed and bends its path.

Common mistake

"Moving things naturally slow down and stop." On Earth they usually do, but only because friction and air act on them. Take those away, and nothing slows down.

Where it leads