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.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
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.
Model answer (4 marks)
Speed is a scalar quantity – it has magnitude only and no direction.
Velocity is a vector quantity – it has both magnitude and direction. As the planet moves round its orbit the direction of its velocity vector continuously changes, so its velocity changes even though its speed is constant.
A change in velocity means the planet is accelerating.
By Newton’s first and second laws a resultant force must act on the planet; this force is directed towards the star and provides the centripetal acceleration needed for the circular orbit.
Velocity is a vector quantity – it has both magnitude and direction. As the planet moves round its orbit the direction of its velocity vector continuously changes, so its velocity changes even though its speed is constant.
A change in velocity means the planet is accelerating.
By Newton’s first and second laws a resultant force must act on the planet; this force is directed towards the star and provides the centripetal acceleration needed for the circular orbit.
Examiner tips
- State that speed is scalar and velocity is vector; mention direction change. Show that constant speed + changing direction = acceleration. Link acceleration to a force via Newton’s laws and specify centripetal direction.
Common mistakes
- Confusing speed with velocity; saying the planet has constant velocity. Forgetting to mention that the force is centripetal (towards the star).
Mark scheme (4 marks)
- Speed is a scalar quantity (it has magnitude only / no direction)
- 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
- A change in velocity means the planet is accelerating
- 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 in this question
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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