Pearson Edexcel · GCSE · Physics
Course version: 1PH0 · Version from Sept 2016
7.6P Explain for circular orbits how the force of gravity can lead to…
Full specification wording
Topic 7 – Astronomy: 7.6P Explain for circular orbits how the force of gravity can lead to changing velocity of a planet but unchanged speed
In a circular orbit the gravitational force from the central body always points towards that body, at right angles to the planet's motion.
Checked against the Pearson Edexcel 1PH0 specification (opens in a new tab) on .
- Point 6 of 18
Study notes
From the checked teaching notes for this specification point.
The idea
In a circular orbit the gravitational force from the central body always points towards that body, at right angles to the planet's motion. Velocity is a vector: it has both size (speed) and direction. Because the direction of travel changes continuously around the circle, the velocity changes even though the speed stays the same. The gravitational force provides the centripetal force that keeps the planet on its curved path. A correct answer therefore separates speed (a scalar, unchanged) from velocity (a vector, changing because direction changes) and links the change to the inward pull of gravity acting perpendicular to the motion.
What earns the marks
- States that velocity is a vector with both magnitude and direction, whereas speed is a scalar.
- Explains that the gravitational force acts towards the central body, at right angles to the planet's motion.
- Concludes that the direction of motion changes continuously, so velocity changes while speed stays constant.
- Identifies gravity as the centripetal force that keeps the planet on its circular path.
Common mistakes and how to put them right
Says the speed changes around a circular orbit.
Put it right by stating that speed is constant and only direction changes.
Treats velocity and speed as the same quantity.
Put it right by defining velocity as a vector with direction.
Places the gravitational force along the direction of motion.
Put it right by directing it towards the central body, perpendicular to the motion.
Exam tips
- Check whether the option distinguishes velocity (vector) from speed (scalar).
- Look for the statement that gravity acts towards the central body, at right angles to motion.
- Reject options claiming the planet speeds up or slows down on a circular orbit.
Try the lesson
The start of the Topic 7 – Astronomy: 7.6P Explain for circular orbits how the force of gravity can lead to changing velocity of a planet but unchanged speed lesson, made from checked teaching notes for this specification point.
Have a go first
A planet in a circular orbit is momentarily moving due north. A short time later it is moving due west. What has the gravitational force from the star done to the planet's motion?
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