Unit 3, lesson 15. About 15 minutes.

Orbits

Why doesn't the Moon fall down?

Play

Getting the experiment ready…

Explain

The Moon is always falling around us.

It moves so fast it keeps missing Earth.

Imagine a giant cannon on a very tall mountain. Fire it slowly, and the ball soon falls to the ground.

Fire it faster and it lands further away. Fire it fast enough, and the ground curves away just as quickly as the ball falls.

The ball never lands. It keeps falling around the Earth. That is an orbit, and the Moon does the same thing.

An orbit is free fall combined with sideways speed. Gravity constantly changes the direction of the velocity.

For a circular orbit, v = √(GM / r). Too slow and the orbit hits the Earth. Faster gives an ellipse, and at about 1.4 times the circular speed the object escapes.

Worked example: at a height of 1000 km, r = 7371 km. So v = √(398 600 / 7371) ≈ 7.4 km/s.

Fire sideways
Gravity curves the path
Earth curves away: an orbit

Want the full story?

Astronauts on the space station feel weightless because they are in constant free fall, not because gravity has gone. At that height, gravity is still about 90% as strong as on the ground. The sim speeds up time by 240 times and leaves out air, and a real cannon couldn't work in the thick lower air.

The Formula

orbit speed = square root of (Earth's gravity number ÷ distance from Earth's centre)

v = \sqrt{\frac{G M}{r}}

$v$: the speed needed for a circular orbit, measured in kilometres per second (km/s)
$G M$: the gravitational constant × Earth's mass, 398 600 for Earth, measured in cubic kilometres per second squared (km³/s²)
$r$: the distance from Earth's centre, which is 6371 km plus the height, measured in kilometres (km)

Try the numbers

Height above the ground: 1,000 km

Orbit speed: 7.35 km/s

Try it at home

Ask a grown-up to help

You need a soft ball, a sock, some string.

You need: a soft ball, a sock, some string.

Swing a soft ball in a circle.

Tie a soft ball in a sock, and tie string to the sock.
In a clear space, swing it around in a circle.
Feel the string pulling inward.
Let go, and see which way the ball goes.

What's happening: the string pulls inward just as gravity does for a satellite. Without the pull, the ball flies off in a straight line.

Physics is life

Your body

You feel floaty when you go over a hill.

In the falling feeling at the top of a swing, you are in free fall for a moment, just like an astronaut.

Daily life

Satellites give us TV and maps.

Satellites in orbit carry phone calls, weather pictures and sat nav to your home.

Nature

Moons go around planets.

Many planets have moons. Jupiter has dozens of them in orbit.

Space

The Moon goes around the Earth.

The Moon takes about 27 days to orbit the Earth, moving at about 1 km every second.

Quick check

Question 1 of 3

Why doesn't the Moon fall to Earth?

Words to know

Orbit: The curved path of one thing around another, held by gravity.

Orbit: The curved path of one thing around another, held by gravity.
Satellite: Something that orbits a planet, like the Moon or a space station.
Gravity: The pull between all things that have mass.

Meet the scientist

Next up

Newton's law of universal gravitation