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Satellites stay in orbit because gravity provides the inward (centripetal) force that keeps them moving in a circle. Free GCSE Physics notes and 14 FAQs.
Circular Orbits and Gravity is part of Solar system; stability of orbital motions; satellites (physics only) in GCSE AQA Higher Physics.
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Satellites stay in orbit because gravity provides the inward (centripetal) force that keeps them moving in a circle. Free GCSE Physics notes and 14 FAQs.
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Use this lesson to stay inside Solar system; stability of orbital motions; satellites (physics only) while connecting the detail back to the wider Space physics (physics only) topic.
Read the core explanations from Circular Orbits and Gravity before testing yourself from memory.
Picture the International Space Station whizzing around Earth at 17,500 mph. It's continuously falling towards Earth, yet it never hits the ground. This is pure physics at work.
The key insight is that the ISS is moving so fast sideways that as it falls towards Earth, the curved surface of our planet falls away beneath it. It's in permanent free fall, but the ground keeps moving out of the way.
This same principle governs every circular orbit in our solar system. From the Moon around Earth to Earth around the Sun, gravity provides exactly the right force to keep objects moving in stable circles.
When something moves in a circle, it needs a force pulling it inwards constantly. Without this inward force, the object would fly off in a straight line due to inertia. Think of whirling a ball on a string - the string tension provides the centripetal force.
For satellites and planets, gravity acts as the inward force that keeps them in orbit.
Here's the crucial point: the centripetal force changes the direction of velocity constantly, but not the speed. This means an orbiting object has changing velocity (because direction changes) but constant speed.
For circular orbits, gravity provides the inward force needed to keep the object moving in a circle.
Consider Earth orbiting the Sun. The Sun's massive gravitational field pulls Earth inwards with a force of about 3.5 × 10²² N. This enormous force doesn't pull Earth into the Sun because Earth is moving sideways at 30 km/s. The result is a stable elliptical orbit (nearly circular).
Artificial satellites orbit Earth and can be used for communication and weather monitoring.
Here's where orbital mechanics gets interesting. For any stable circular orbit, the speed and radius are intimately connected. You can't just choose any speed at any distance - physics dictates the relationship.
A satellite needs the right speed to remain in orbit.
A change in speed can change the size of an orbit.
Worked example: Orbiting Satellites
Explain how gravity keeps a satellite in orbit.
Answer: A satellite must move fast enough to stay in orbit.
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A stable orbit needs the right speed so that gravity can keep the object moving in a circle.
A satellite needs the right speed to remain in orbit.
Satellites can be adjusted to keep them in orbit.
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Satellites are used for navigation and communication.
Geostationary satellites stay above the same point on Earth and are useful for communication and weather monitoring.
Space missions to other planets use orbital mechanics for efficiency. The Voyager probes used gravitational slingshots around Jupiter and Saturn to gain speed and change direction. By carefully timing their approach, engineers used each planet's gravity to accelerate the spacecraft towards the next target.
Circular orbit questions often test your understanding that gravity keeps satellites in orbit.
When tackling orbit questions, remember that gravity keeps satellites in orbit.
Watch out for the common misconception that satellites need the right speed to stay in orbit.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Earth orbits the Sun at about 30 km/s. Its sideways motion, together with the Sun's gravitational pull of about 3.5 × 10²² N, gives a stable, nearly circular elliptical orbit.
The International Space Station whizzes around Earth at about 17,500 mph. At an orbital radius of 6.8 × 10⁶ m, 400 km from Earth's surface, its orbital speed works out at about 7670 m/s, so it keeps falling around Earth rather than into it.
Centripetal force is the force directed towards the centre of a circular path that keeps an object moving in a circle, measured in newtons (N). For a ball whirled on a string it is the tension; for satellites and planets it is gravity.
Satellites stay in orbit because gravity provides the inward (centripetal) force that keeps them moving in a circle. A satellite is continuously falling towards Earth, but it moves sideways so fast that the curved surface of the planet falls away beneath it.
In a circular orbit, the centripetal force constantly changes the direction of the velocity but not the speed. Because velocity includes direction, an orbiting object has a changing velocity even though its speed stays constant.
The International Space Station is continuously falling towards Earth, but it moves sideways at about 17,500 mph. As it falls, the curved surface of Earth falls away beneath it, so it stays in permanent free fall and never hits the ground.
A satellite needs the right speed to remain in orbit, and a change in speed can change the size of its orbit. For a stable circular orbit, the speed and radius are linked, so a satellite cannot have just any speed at any distance.
Gravity provides the centripetal force for orbits: it acts as the inward force that keeps satellites, moons and planets moving in circles. On a ball whirled on a string, the string tension provides this force instead.
Without an inward centripetal force, an object moving in a circle would fly off in a straight line due to inertia. Gravity provides this force for orbiting objects.
The Sun's gravitational field pulls Earth inwards with a force of about 3.5 × 10²² N. This enormous force does not pull Earth into the Sun because Earth is moving sideways at 30 km/s.
A stable orbit is an orbit where gravity keeps the object moving in a circle. It needs the right speed, so that gravity can keep the object on its circular path.
The Voyager probes used gravitational slingshots around Jupiter and Saturn to gain speed and change direction. By carefully timing each approach, engineers used each planet's gravity to accelerate the spacecraft towards the next target.
An orbiting object in a circular orbit has a constant speed but a changing velocity, because the centripetal force constantly changes the direction of its velocity, not its speed.
The orbital radius of a satellite is Earth's radius plus its altitude: for a satellite 400 km up, r = 6400 + 400 = 6800 km, or 6.8 × 10⁶ m.