# A planet orbits the Sun in a circular orbit. The gravitational force acting on the planet is a non-contact force. Explain how this gravitational force keeps the planet in a circular orbit, and state what would happen to the planet's motion if the gravitational force were suddenly removed.

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

## Mark scheme (4 marks)

1. Gravity is a non-contact force that acts towards the Sun (towards the centre of the orbit)
2. This force continuously changes the direction of the planet's velocity (without changing its speed)
3. Because velocity is a vector quantity, a change in direction means the planet is continuously accelerating (even at constant speed)
4. If the gravitational force were removed, the planet would continue moving in a straight line (at constant velocity / tangent to the orbit), in accordance with Newton's first law

## Key terms

- [non-contact force](https://www.gradenine.co.uk/glossary/non-contact-force)
- [gravitational force](https://www.gradenine.co.uk/glossary/gravitational-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-the-sun-in-a6db93b4) · Published by Druglandscape Ltd.