Explain why a satellite in a circular orbit around the Earth does not need a continuous thrust force to maintain its orbital speed, yet the satellite is still accelerating.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The gravitational force provides the required centripetal force, acting towards the centre of the Earth and perpendicular to the satellite’s velocity.
Because the force is perpendicular to the velocity, it changes only the direction of motion, not the speed, so no continuous thrust is required to keep the speed constant.
The satellite still accelerates because acceleration is the rate of change of velocity (a vector). The continuous change in the direction of the velocity vector means the satellite is accelerating even though its speed remains constant.
Because the force is perpendicular to the velocity, it changes only the direction of motion, not the speed, so no continuous thrust is required to keep the speed constant.
The satellite still accelerates because acceleration is the rate of change of velocity (a vector). The continuous change in the direction of the velocity vector means the satellite is accelerating even though its speed remains constant.
Examiner tips
- Show the force is centripetal and perpendicular to velocity; explain no work is done; state that speed is constant but velocity changes; link this to acceleration as a vector change.
- Use the words "centripetal", "perpendicular", "speed", "velocity", "acceleration" exactly as in the mark scheme.
Common mistakes
- Confusing speed with velocity and claiming the satellite is not accelerating; stating that gravity does work on the satellite; omitting that the force is perpendicular to the velocity.
Mark scheme (4 marks)
- The gravitational force acts as a centripetal force, directed towards the centre of the Earth / perpendicular to the velocity of the satellite.
- A force perpendicular to velocity changes the direction of motion but does not do work on / change the speed of the satellite.
- Because no thrust is needed to maintain speed (gravity alone sustains the orbit), no engine force is required.
- Acceleration is the rate of change of velocity (a vector); the satellite's velocity direction changes continuously, so it accelerates even though its speed is constant.
Key terms in this question
Related
- All IB DP Physics Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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