A satellite orbits the Earth in a circular orbit. Although the gravitational force acting on the satellite is a non-contact force, it does not do work on the satellite, and the satellite maintains a constant orbital speed. Explain why the gravitational force does not change the satellite's speed, and explain what effect the gravitational force does have on the satellite's motion.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Satellites used for weather monitoring orbit the Earth in circular orbits at a constant orbital speed. Gravity is the only force acting on such a satellite.
Model answer (4 marks)
The gravitational force acts perpendicular to the satellite’s instantaneous direction of motion.
Because the force has no component along the direction of velocity, it cannot change the magnitude of the velocity; the speed remains constant.
However, the force continually changes the direction of the velocity vector.
Thus the satellite experiences a centripetal acceleration that keeps it in circular motion.
Because the force has no component along the direction of velocity, it cannot change the magnitude of the velocity; the speed remains constant.
However, the force continually changes the direction of the velocity vector.
Thus the satellite experiences a centripetal acceleration that keeps it in circular motion.
Examiner tips
- Use the phrase "perpendicular to the velocity" to show understanding of force direction. Explain that a force only changes speed if it has a component along the velocity. Mention that the changing direction of velocity is what constitutes acceleration.
- common_mistakes
- :
- Confusing force direction with velocity direction. Saying the force does work or changes speed. Forgetting to mention that acceleration is centripetal.
Mark scheme (4 marks)
- The gravitational force acts perpendicular to (at right angles to) the satellite's direction of motion / velocity
- A force can only change speed if it has a component in the direction of motion / because there is no component of force in the direction of motion, the speed does not change
- The gravitational force continuously changes the direction of the satellite's velocity / motion
- Because velocity is a vector quantity and its direction is changing, the satellite is accelerating / the gravitational force causes a centripetal acceleration
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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