A satellite orbits the Earth in a circular path at a constant speed. Explain why the satellite continuously changes direction even though no engine thrust is acting along its direction of travel, and describe the direction and nature of the force responsible for keeping it in circular motion.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Artificial satellites travel around the Earth in circular orbits. Although their speed remains constant, they are not travelling in a straight line.
Model answer (5 marks)
A satellite in a circular orbit has a velocity that is always tangent to the orbit. Because the direction of the velocity vector changes continuously, the satellite’s velocity is constantly changing.
To change the velocity a net force is required. The only force acting on the satellite is the gravitational attraction between the satellite and the Earth.
Gravity pulls the satellite toward the centre of the Earth, i.e. toward the centre of the circular orbit. This inward force is the centripetal force.
Since the centripetal force is always perpendicular to the velocity, it changes the direction of the velocity but does no work, so the speed (and kinetic energy) remains constant.
To change the velocity a net force is required. The only force acting on the satellite is the gravitational attraction between the satellite and the Earth.
Gravity pulls the satellite toward the centre of the Earth, i.e. toward the centre of the circular orbit. This inward force is the centripetal force.
Since the centripetal force is always perpendicular to the velocity, it changes the direction of the velocity but does no work, so the speed (and kinetic energy) remains constant.
Examiner tips
- Show the velocity vector is changing by noting it is always tangent to the orbit.
- Explain that a net force is needed to change velocity – mention gravity as the only force.
- State that gravity acts toward the centre (centripetal) and is perpendicular to velocity.
- Mention that the force does no work, keeping the speed constant.
Common mistakes
- Confusing centripetal force with centrifugal force or describing the force as acting along the direction of travel.
- Failing to explain that the force is perpendicular to velocity and therefore does no work.
- Omitting the point that the speed remains constant because the force does no work.
Mark scheme (5 marks)
- The satellite's velocity is constantly changing because its direction is constantly changing
- A resultant / unbalanced force is needed to change the velocity (i.e. to accelerate the satellite)
- The force acting on the satellite is gravity (gravitational attraction between the satellite and the Earth)
- The force acts towards the centre of the Earth / towards the centre of the circular orbit (centripetal direction)
- Because the force is always perpendicular to the velocity / directed inward, it changes direction but does no work on the satellite, so the speed (and therefore kinetic energy) remains constant
Key terms in this question
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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