AQA · GCSE · Physics
Course version: 8463 · Version from Sept 2016
Gravity provides the force that allows planets and satellites (both…
Full specification wording
Orbital motion, natural and artificial satellites: Gravity provides the force that allows planets and satellites (both natural and artificial) to maintain their circular orbits.
An object in orbit is falling all the time, and the force doing the pulling is gravity.
Checked against the AQA 8463 specification (opens in a new tab) on .
- Point 1 of 3
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
An object in orbit is falling all the time, and the force doing the pulling is gravity. Gravitational attraction between the Earth and a satellite acts along the line joining them, pulling the satellite towards the centre of its orbit. That force is at right angles to the direction the satellite is moving, so it does not speed the satellite up or slow it down; it bends the path into a circle instead. Take the gravitational force away and the satellite would carry straight on at a constant velocity and leave the orbit. The same force holds the Moon to the Earth and holds the planets to the Sun, which is why nothing in orbit needs an engine to stay there.
What earns the marks
- Gravity, or gravitational attraction, being the force that keeps an object in orbit.
- That force acting towards the centre of the orbit.
- The force being at right angles to the direction of motion.
- Stating that without that force the object would travel in a straight line.
Common mistakes and how to put them right
Saying that satellites are held in orbit by their engines, or by a force pushing them forwards.
Drawing the gravitational force backwards along the orbit, which would slow the satellite down.
Claiming there is no gravity in space, which would leave nothing to bend the path.
Writing gravity with no statement of which two objects are attracting each other.
Exam tips
- Draw the force arrow pointing at the centre of the orbit, never along the path.
- Name both objects that attract each other, for example the Earth and the satellite.
- If asked what would happen without gravity, say a straight line at constant speed, not that it would stop.
Try the lesson
The start of the Orbital motion, natural and artificial satellites: Gravity provides the force that allows planets and satellites (both natural and artificial) to maintain their circular orbits. lesson, made from checked teaching notes for this specification point.
Have a go first
What force keeps a planet in a circular orbit around a star?
Listen
An object in orbit is falling all the time, and the force doing the pulling is gravity. Gravitational attraction between the Earth and a satellite acts along the line joining them, pulling the satellite towards the centre of its orbit. That force is at right angles to the direction the satellite is moving, so it does not speed the satellite up or slow it down; it bends the path into a circle instead. Take the gravitational force away and the satellite would carry straight on at a constant velocity and leave the orbit. The same force holds the Moon to the Earth and holds the planets to the Sun, which is why nothing in orbit needs an engine to stay there. A strong answer earns marks for these points. gravity, or gravitational attraction, being the force that keeps an object in orbit. that force acting towards the centre of the orbit. the force being at right angles to the direction of motion. stating that without that force the object would travel in a straight line. Exam tip: Draw the force arrow pointing at the centre of the orbit, never along the path.
See it
Key words
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gravitational force
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circular orbit
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force towards the centre
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at right angles to the motion
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See every definition
- gravitational force
- The force that pulls a satellite towards the centre of its orbit, acting along the line joining the Earth and the satellite.
- circular orbit
- The path of a planet or satellite that is bent into a circle by a force at right angles to its motion.
- force towards the centre
- The gravitational attraction that pulls a satellite towards the centre of its orbit rather than along its path.
- at right angles to the motion
- The direction of the gravitational force on an orbiting object, so it does not speed the object up or slow it down.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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