The Moon orbits the Earth at a constant orbital speed. Explain why the Moon is continuously accelerating even though its orbital speed does not change.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The Moon travels in a nearly circular orbit around the Earth, completing one orbit approximately every 27 days. Its orbital speed remains roughly constant throughout its orbit.
Model answer (4 marks)
Velocity is speed in a given direction, so velocity is a vector quantity (has both magnitude and direction). The direction of the Moon’s motion is continuously changing as it orbits. A change in direction means the velocity changes, and acceleration is the rate of change of velocity. There is a resultant (gravitational) force acting on the Moon towards the Earth, which causes this acceleration.
Examiner tips
- Use the definition of velocity as a vector; mention magnitude and direction.
- Explain that changing direction alone changes velocity even if speed is constant.
- Show that acceleration is the rate of change of velocity, not just speed.
- Mention the gravitational force as the source of the acceleration.
Common mistakes
- Saying the Moon is not accelerating because speed is constant; ignores direction.
- Confusing acceleration with speed change; not recognising vector nature of velocity.
- Failing to mention the gravitational force as the cause of the acceleration.
Mark scheme (4 marks)
- Velocity is speed in a given direction, so velocity is a vector quantity (has both magnitude and direction)
- The direction of the Moon's motion is continuously changing as it orbits
- A change in direction means the velocity changes, and acceleration is the rate of change of velocity
- There is a resultant (gravitational) force acting on the Moon towards the Earth, which causes this acceleration
Related
- All Pearson Edexcel International GCSE Physics (4PH1) revision notes →
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- Decode the mark scheme abbreviations →
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