# A planet orbits a star at a constant orbital speed. State what type of quantity orbital speed is, and explain why the planet is not travelling at constant velocity during its orbit. Then explain what this tells us about the resultant force acting on the planet.

> Pearson Edexcel International GCSE Physics (4PH1) — 1.4 Astronomy · Explain · 4 marks

> A planet in a distant solar system completes one full orbit of its star in 365 days, maintaining the same orbital speed throughout its journey.

## Mark scheme (4 marks)

1. Speed is a scalar quantity (it has magnitude only / no direction)
2. Velocity is a vector quantity, so it has both magnitude and direction; as the planet moves around its orbit, its direction continuously changes, so its velocity continuously changes even though its speed is constant
3. A change in velocity means the planet is accelerating
4. By Newton's first law / Newton's second law, a resultant force must be acting on the planet (directed towards the star / centripetally)

## Key terms

- [velocity](https://www.gradenine.co.uk/glossary/velocity)
- [resultant force](https://www.gradenine.co.uk/glossary/resultant-force)

## Related

- [Revision notes for Pearson Edexcel International GCSE Physics (4PH1)](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-planet-orbits-a-star-at-a9e9b951) · Published by Druglandscape Ltd.