# Explain why a planet in a circular orbit around a star continues to travel in that circular path rather than moving off in a straight line.

> Eduqas A-Level Physics — 4.3 Orbits and the wider universe · Explain · 5 marks

> Planets in our Solar System travel in nearly circular orbits around the Sun. Without a force acting on them, an object in space would travel in a straight line at constant speed.

## Mark scheme (5 marks)

1. Gravity / gravitational force acts between the planet and the star
2. The gravitational force acts towards the star / is directed inward (centripetal direction)
3. This force continuously changes the direction of the planet's velocity / motion
4. Without this force the planet would travel in a straight line (Newton's first law / inertia)
5. The speed / magnitude of velocity stays constant (so the planet remains in orbit at the same distance)

## Key terms

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

## Related

- [Revision notes for Eduqas A-Level Physics](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-planet-in-a-c5a16ca4) · Published by Druglandscape Ltd.