# A satellite orbits the Earth in a circular path at a constant speed. Explain why the satellite continuously changes direction even though no engine thrust is acting along its direction of travel, and describe the direction and nature of the force responsible for keeping it in circular motion.

> WJEC A-Level Physics (Wales) — 1.5 Circular motion (A-Level only) · Explain · 5 marks

> Artificial satellites travel around the Earth in circular orbits. Although their speed remains constant, they are not travelling in a straight line.

## Mark scheme (5 marks)

1. The satellite's velocity is constantly changing because its direction is constantly changing
2. A resultant / unbalanced force is needed to change the velocity (i.e. to accelerate the satellite)
3. The force acting on the satellite is gravity (gravitational attraction between the satellite and the Earth)
4. The force acts towards the centre of the Earth / towards the centre of the circular orbit (centripetal direction)
5. Because the force is always perpendicular to the velocity / directed inward, it changes direction but does no work on the satellite, so the speed (and therefore kinetic energy) remains constant

## Key terms

- [circular motion](https://www.gradenine.co.uk/glossary/circular-motion)

## Related

- [Revision notes for WJEC A-Level Physics (Wales)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-satellite-orbits-the-earth-in-161d4ee3) · Published by Druglandscape Ltd.