GCSE · Physics · AQA · Spec 8463

Orbital motion and satellites

Satellites circle above you for years, with no engine pushing them round. So what keeps them going? Gravity — the same pull that keeps your feet on the floor.

Physics · Orbits

What keeps a satellite going round?

View: 1 · Gravity switched off. Showing 3 layers: Earth, Circular orbit, Gravity switched off

View

start at 1, then 2, then 3

Imagine switching Earth's gravity off. Nothing pulls on the satellite, so nothing turns it: it carries straight on in the direction it was already moving, and leaves the dashed circle for good.

Step through the three views in order. Watch the arrows: what changes, and what stays exactly the same?

Planets, moons and satellites

Same force, different centres

Gravity doesn't only hold satellites in orbit. Sort each statement into the right circles: planets, moons, artificial satellites — some belong in more than one, and one belongs in none.

A Planets
B Moons
C Artificial satellites

Orbits the Sun
Orbits a planet
Called a natural satellite
Made and launched by people
A natural object — nobody made it
Gravity provides the force that keeps its orbit circular
Needs a forward push to keep going round

Higher

What do you really think?

Steady speed — so is anything acting on it?

A satellite goes round Earth in a circular orbit. Its speed never changes.

Which idea is closest to what you think right now? Be honest — that's how this works.

Nothing is acting on it. It's up in space, and there's no gravity out there.Its speed isn't changing, so the forces on it must be balanced.Something must be pushing it forward along its path, or it would stop.Gravity pulls it towards Earth all the time. That pull changes its direction, so its velocity keeps changing even though its speed stays the same.

How sure are you?

I'm sureI think soI'm guessing

Nothing is acting on it. It's up in space, and there's no gravity out there.

This is a common misconception

Gravity doesn't stop at the edge of space. If nothing acted on the satellite, it would carry straight on and leave its orbit — that's view 1 in the diagram. The only reason it keeps curving round is that Earth's gravity is pulling on it the whole time.

Its speed isn't changing, so the forces on it must be balanced.

This is a common misconception

This is the one that catches most people. Balanced forces would mean no change in speed AND no change in direction — the satellite would go in a straight line. But its direction changes every moment, so its velocity is changing. A changing velocity means it's accelerating, and that needs an unbalanced force: gravity.

Something must be pushing it forward along its path, or it would stop.

This is a common misconception

No forward push is needed to keep it going round. The force that keeps it in orbit is gravity, and gravity points towards the centre of Earth — at right angles to the way it's moving. The job isn't to push it along; it's to keep turning it.

Gravity pulls it towards Earth all the time. That pull changes its direction, so its velocity keeps changing even though its speed stays the same.

This matches the evidence

Exactly. Gravity pulls towards the centre of Earth, at right angles to the motion, so it changes the satellite's direction but not its speed. Velocity is speed in a given direction — so a new direction means a new velocity, and the satellite is accelerating the whole way round.

Constant speed, changing direction: a satellite's velocity changes all the time, and gravity is the force doing it.

Higher

Put it in writing

Constant speed, changing velocity — explain it

A satellite travels in a circular orbit around Earth at a constant speed. Explain how the satellite can have a constant speed but a changing velocity. [3 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

Gravity (the gravitational force from Earth) acts on the satellite, towards the centre of Earth. (1 mark)
This force changes the satellite's direction but not its speed. (1 mark)
Velocity depends on direction as well as speed, so a changing direction means a changing velocity. (1 mark)

Model answer

Earth's gravity pulls on the satellite towards the centre of Earth, at right angles to the way it is moving. This pull changes the satellite's direction but not its speed. Velocity is speed in a given direction, so because the direction keeps changing, the velocity keeps changing even though the speed stays the same.

Name the force, say what it changes (direction, not speed), then link direction to velocity. Those three moves are the whole explanation.

Higher

Speed and orbit size

Reason it through

Why must a satellite's orbital radius change if its speed changes?

Link 1 of 4

First link · your turn

Start with what gravity is doing. At any moment, what does its pull do to the satellite's path?

Locked — reveal the link above first

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

What you need to know

Gravity provides the force that keeps planets, moons and artificial satellites in their circular orbits.
Planets orbit the Sun, moons are natural satellites that orbit planets, and artificial satellites orbit Earth.
An artificial satellite orbits Earth because of the gravitational attraction between the satellite and Earth.
Higher: a satellite's speed along its circular orbit stays constant, but its velocity keeps changing because its direction keeps changing.
Higher: for a stable orbit, if a satellite's speed changes, its orbital radius must change.

The big picture

Gravity is the force that keeps planets, moons and artificial satellites in their circular orbits. Planets orbit the Sun, moons are natural satellites that orbit planets, and artificial satellites orbit Earth. Higher: gravity changes a satellite's direction but not its speed, so its velocity keeps changing; and if its speed changes, its orbital radius must change for it to stay in a stable orbit.

Key points

1One force does the job: gravity pulls an orbiting body towards the centre of whatever it orbits, all the time.

2Take gravity away and the body would carry on in a straight line — the pull is what bends its path into a circle.

3Same force, different centres: the Sun for planets, a planet for its moons, Earth for artificial satellites.

4Higher: gravity makes a satellite accelerate by changing its direction, not its speed.

5Higher: speed and orbital radius go together — change the speed and, for a stable orbit, the radius must change too.

.we-_R_18slubsnpfiv5ubrb_ [data-we-step]{display:flex !important}.we-_R_18slubsnpfiv5ubrb_ [data-we-solution]{display:block !important}.we-_R_18slubsnpfiv5ubrb_ [data-we-reveal]{display:none !important}

Worked example

Problem

The Moon orbits Earth. Describe the force that keeps the Moon in its orbit, and show on a sketch which way it acts.

⚠ Watch out

Saying there's no gravity in space, so nothing acts on a satellite. Gravity is exactly what keeps a satellite in orbit — take it away and the satellite would carry on in a straight line and leave its orbit.

🧠

Memory hook

Pulled in, never pushed along: gravity points to the centre and bends the path round.

✓

Check yourself

Cover the page. What does a planet orbit, and a moon? Which force keeps a satellite going round Earth, and which way does it point? Higher: why is a steady-speed satellite's velocity changing?

Flashcards

(11)

Which force keeps planets and satellites in their circular orbits?

Gravity. It provides the force that maintains the circular orbit of planets, moons and artificial satellites.

What do planets orbit?

The Sun.

What is a moon?

A natural satellite that orbits a planet.

What is an artificial satellite, and what does it orbit?

A satellite made and launched by people. Artificial satellites orbit Earth.

Why does an artificial satellite orbit Earth?

Because of the gravitational attraction between the satellite and Earth.

Does gravity pull an orbiting satellite along its path, or across it?

Across it: inward, towards Earth's middle. That's why it turns the satellite instead of pushing it along.

If gravity suddenly stopped acting on a satellite, what would it do?

Carry on in a straight line in the direction it was moving at that moment, leaving its orbit.

What is the difference between speed and velocity?

Speed is how fast something moves. Velocity is speed in a given direction, so it changes if the direction changes.

Higher: what happens to a satellite's speed along its circular orbit?

It stays constant.

Higher: why does an orbiting satellite's velocity keep changing?

Gravity keeps changing its direction, and a change of direction is a change of velocity — so the satellite is accelerating.

Higher: a satellite's speed changes. What must happen for it to stay in a stable orbit?

Its orbital radius must change.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

Learning with Lightbulb is opening soon

You can use this lesson now. Join the waitlist and we'll let you know when the full Lightbulb experience is ready.

Keep me posted

Orbital motion and satellites

Same force, different centres

Steady speed — so is anything acting on it?

Constant speed, changing velocity — explain it

More AQA GCSE Physics topics

Helpful guides for parents and students