Orbital Motions and Satellites
Understand Orbital Motions and Satellites for GCSE Physics with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. A separate (triple) physics topic: gravity provides the force that keeps objects in orbit, and for a stable circular orbit a smaller radius means a higher speed.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
Gravity provides the force that allows planets and satellites to maintain their circular orbits. This is a Triple-only topic.
An object in a circular orbit moves at a constant speed, but its velocity is continually changing because its direction is continually changing. A changing velocity means the object is accelerating, and that acceleration is caused by the gravitational force acting towards the centre of the orbit.
For a stable orbit at a given radius, there is only one possible speed. If the speed changes, the radius must change too: a faster speed requires a smaller orbital radius. This is why satellites in low orbits travel faster than those further out.
Revision notes
Gravity provides the force
Gravity provides the force that allows planets and satellites to maintain circular orbits.
The gravitational force acts towards the centre of the orbit, continually changing the direction of motion without changing the speed.
Constant speed, changing velocity
An object in a circular orbit travels at a constant speed but its velocity changes continually.
Velocity is a vector, and the direction is always changing. A changing velocity means the object is accelerating, even though its speed is constant.
Speed and radius
For a circular orbit at a given radius, there is only one possible speed.
If the speed changes, the radius must change. A faster satellite orbits at a smaller radius; a slower one orbits further out. This is why low satellites complete an orbit more quickly.
Key points
- Gravity provides the force for circular orbits.
- The force acts towards the centre of the orbit.
- Speed is constant but velocity changes.
- Changing velocity means the object accelerates.
- Each orbital radius has only one possible speed.
- A faster speed requires a smaller radius.
Worked examples
Example 1
Explain why an object in a circular orbit is accelerating even though its speed is constant. [3 marks]
Model answer
Velocity is a vector, including both speed and direction (1 mark). In a circular orbit the direction is continually changing, so the velocity is continually changing (1 mark). A changing velocity means the object is accelerating (1 mark).
Example 2
State what happens to the orbital radius if a satellite's speed increases. [2 marks]
Model answer
The radius must decrease (1 mark), because for a stable orbit there is only one possible speed at each radius and a faster speed corresponds to a smaller radius (1 mark).
Example 3
Name the force that keeps a planet in orbit around the Sun. [1 mark]
Model answer
The gravitational force between the planet and the Sun (1 mark).
Common mistakes
Saying the speed changes in a circular orbit.
The speed is constant; it is the velocity that changes because of direction.
Saying no force acts in orbit.
Gravity acts continuously towards the centre of the orbit.
Getting the speed-radius relationship backwards.
Faster means a smaller radius, not larger.
Saying the force acts along the direction of travel.
It acts towards the centre, perpendicular to the motion.
Exam tips
- Explain acceleration through changing direction.
- Say the force acts towards the centre of the orbit.
- Learn that faster orbits have smaller radii.
- Remember this is a Triple-only topic.
Key terms
- Orbit
- The curved path of an object around a larger body.
- Circular orbit
- An orbit of constant radius and constant speed.
- Velocity
- Speed in a given direction, changing throughout an orbit.
- Gravitational force
- The force acting towards the centre of the orbit.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.