A satellite travels at constant speed in a circular orbit around the Earth. Explain why the satellite is accelerating even though its speed does not change, and describe the direction of both the resultant force and the acceleration acting on the satellite.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Satellites used for communication travel at constant speed in circular orbits above the Earth's surface.
Model answer (5 marks)
Velocity is a vector; it has both magnitude (speed) and direction.
The satellite’s direction of motion is continually changing as it moves round the circle.
A change in velocity (even if the speed is constant) means the satellite is accelerating.
The resultant force on the satellite is directed towards the centre of the Earth, i.e. the centre of the circular orbit.
The acceleration is in the same direction as the resultant force – towards the centre of the Earth.
The satellite’s direction of motion is continually changing as it moves round the circle.
A change in velocity (even if the speed is constant) means the satellite is accelerating.
The resultant force on the satellite is directed towards the centre of the Earth, i.e. the centre of the circular orbit.
The acceleration is in the same direction as the resultant force – towards the centre of the Earth.
Examiner tips
- Use the term ‘velocity is a vector’ to show understanding of vector nature.
- Explain that changing direction alone gives acceleration – this earns the full 5 marks.
- Mention the direction of force and acceleration as ‘towards the centre of the Earth’ to match the scheme.
Common mistakes
- Saying the satellite is not accelerating because speed is constant – ignores vector change.
- Forgetting to state the direction of the force/acceleration – loses marks.
- Using ‘towards the Earth’ without specifying ‘centre of the orbit’ – slightly vague.
Mark scheme (5 marks)
- Velocity is a vector quantity / velocity has both magnitude (speed) and direction
- The direction of the satellite's motion is continuously changing as it travels around the circular orbit
- A change in velocity (even at constant speed) means the satellite is accelerating
- The resultant force acting on the satellite is directed towards the centre of the Earth / towards the centre of the circular orbit
- The acceleration is also directed towards the centre of the Earth / in the same direction as the resultant force
Key terms in this question
acceleration · resultant force · circular orbit
Related
- All Eduqas A-Level Physics revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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