# Explain why a satellite in a circular orbit around the Earth does not need a continuous thrust force to maintain its orbital speed, yet the satellite is still accelerating.

> IB DP Physics Higher Level (2023 syllabus) — D.1 Gravitational fields · Explain · 4 marks

## Mark scheme (4 marks)

1. The gravitational force acts as a centripetal force, directed towards the centre of the Earth / perpendicular to the velocity of the satellite.
2. A force perpendicular to velocity changes the direction of motion but does not do work on / change the speed of the satellite.
3. Because no thrust is needed to maintain speed (gravity alone sustains the orbit), no engine force is required.
4. Acceleration is the rate of change of velocity (a vector); the satellite's velocity direction changes continuously, so it accelerates even though its speed is constant.

## Key terms

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

## Related

- [Revision notes for IB DP Physics Higher Level (2023 syllabus)](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/explain-why-a-satellite-in-a-a3065788) · Published by Druglandscape Ltd.