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.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Gravity is a non‑contact force that acts towards the Sun, i.e. towards the centre of the orbit.
This force continuously changes the direction of the planet’s velocity (without changing its speed). Because velocity is a vector, a change in direction means the planet is continuously accelerating, even though its speed remains constant.
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.
This force continuously changes the direction of the planet’s velocity (without changing its speed). Because velocity is a vector, a change in direction means the planet is continuously accelerating, even though its speed remains constant.
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.
Examiner tips
- Use the phrase ‘non‑contact force that acts towards the Sun’ to show you know the nature of gravity.
- Explain that the force changes the direction of velocity, not its magnitude, to get the full 2 marks for the mechanism.
- State that the planet would move in a straight line at constant speed to earn the 1 mark for the consequence.
Common mistakes
- Saying the planet would ‘stop’ or ‘fall away’ – it would keep moving in a straight line.
- Confusing ‘speed’ with ‘velocity’ – only the direction changes, not the speed.
- Omitting that the motion would be along a tangent to the orbit.”
Mark scheme (4 marks)
- Gravity is a non-contact force that acts towards the Sun (towards the centre of the orbit)
- This force continuously changes the direction of the planet's velocity (without changing its speed)
- Because velocity is a vector quantity, a change in direction means the planet is continuously accelerating (even at constant speed)
- 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 in this question
non-contact force · gravitational force
Related
- All Pearson Edexcel International GCSE Physics (4PH1) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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