Unit 2, lesson 6. About 15 minutes.

Newton's First Law

If you slide a ball through empty space, when will it stop?

Play

Getting the experiment ready…

Explain

Things like to keep doing what they are doing.

A rolling ball keeps rolling until something stops it.

Isaac Newton noticed something surprising. Still things stay still, and moving things keep moving, unless a force changes them.

This habit of keeping on doing the same thing is called inertia.

So why does a rolling ball stop on the grass? Friction! The grass pushes back against the ball and slows it down. On ice there is much less friction, so things slide much further.

In empty space, a moving ball would just keep going.

Newton's first law: an object stays at rest, or keeps moving at a constant velocity, unless a net force acts on it.

Moving does not need a force. Changing motion needs a force. The everyday idea that things stop "by themselves" is wrong: friction and air resistance are forces that stop them.

In the simulation, friction is the only horizontal force. Set it to zero and the speed stays exactly constant, so the distance is simply speed multiplied by time.

Worked example: a puck moves at 6 m/s with no friction for 5 s. It travels 6 × 5 = 30 m.

Want the full story?

The friction values in the simulation are illustrative, because real values depend on the exact materials. Also, "no friction" is an ideal case that never quite happens on Earth. Velocity includes direction, so a constant velocity means both constant speed and a straight line. The first law also needs a suitable frame of reference, one that isn't itself accelerating.

The Formula

distance = speed × time, when no net force acts

d = v \times t \quad (\text{when } F_{\text{net}} = 0)

$d$: the distance travelled, measured in metres (m)
$v$: the speed, which stays the same, measured in metres per second (m/s)
$t$: the time taken, measured in seconds (s)
$F_{\text{net}}$: the net force, which must be zero, measured in newtons (N)

Try the numbers

Speed: 6 m/sTime: 5 s

Distance with no friction: 30 m

Try it at home

Ask a grown-up to help

You need a plastic cup, a playing card or stiff postcard, a large coin.

You need: a plastic cup, a playing card or stiff postcard, a large coin.

Flick the card fast. The coin drops in the cup!

Put the card on top of the cup, and the coin in the middle of the card.
Flick the card sideways, quick and flat.
Watch the coin.

What's happening: the fast flick moves the card, but it barely pulls on the coin. The coin's inertia keeps it in place, then gravity pulls it down into the cup.

Coins are a choking hazard, so keep them away from small children's mouths.

Physics is life

Your body

When a bus stops, your body keeps going.

When a bus brakes, your body keeps moving forward. That's your own inertia.

Daily life

A seatbelt holds you still when a car stops.

Seatbelts stop your body when a car stops suddenly, so that inertia can't throw you forward.

Nature

A rolling rock keeps going until something stops it.

A rolling rock keeps going until friction or another rock stops it.

Space

In space, moving things keep on moving.

The Voyager probes launched in 1977. They are still coasting through space, with no engine pushing them along.

Quick check

Question 1 of 3

Why does a rolling ball stop on the grass?

Words to know

Inertia: The way things keep doing what they are doing unless a force changes it.

Inertia: The way things keep doing what they are doing unless a force changes it.
Friction: A force between surfaces that rub together. It resists sliding.
Speed: How far something goes in each second.
Net force: The total force left over after adding up all the pushes and pulls.

Meet the scientist

Isaac Newton

Next up

Newton's Second Law: F = m × a