OCR · GCSE · Physics
Course version: J249 · Version from Sept 2018
Explain for circular orbits, how the force of gravity can lead to changing…
Full specification wording
Beyond Earth: explain for circular orbits, how the force of gravity can lead to changing velocity of a planet but unchanged speed (qualitative only) (content found only in the physics separate science qualification) (Higher tier only)
For a planet moving in a circular orbit, the gravitational force from the Sun acts towards the centre of the orbit.
Checked against the OCR J249 specification (opens in a new tab) on .
- Point 6 of 8
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
For a planet moving in a circular orbit, the gravitational force from the Sun acts towards the centre of the orbit. Velocity is a vector: it has both a size (speed) and a direction. Because the planet's direction of motion changes continuously around the circle, its velocity changes even though its speed stays constant. The gravitational force is always perpendicular to the planet's motion, so it changes the direction of motion rather than the speed. This is a qualitative explanation only: you need to state that the force acts towards the centre, that the direction of motion changes, and that velocity is a vector so a change of direction means a change of velocity while speed remains the same.
What earns the marks
- States that the gravitational force on the planet acts towards the centre of its circular orbit.
- Explains that the planet's direction of motion changes continuously as it moves around the circle.
- States that velocity is a vector quantity with both size and direction.
- Concludes that a changing direction changes the velocity while the speed stays constant.
- Explains that the gravitational force acts perpendicular to the motion, so it changes direction rather than speed.
Common mistakes and how to put them right
Claims the speed changes because the velocity changes.
Put it right: State that speed stays constant while direction, and so velocity, changes.
Treats velocity and speed as the same quantity.
Put it right: Identify velocity as a vector with direction as well as size.
Places the gravitational force along the direction of motion.
Put it right: State that it acts towards the centre of the orbit.
Says no force acts because the speed is constant.
Put it right: Explain that a centripetal force is needed to change direction.
Exam tips
- Define velocity as a vector before explaining why it changes.
- Link the direction of the gravitational force to the centre of the orbit.
- Use the phrase 'speed stays the same but direction changes' to make the distinction clear.
Try the lesson
The start of the Beyond Earth: explain for circular orbits, how the force of gravity can lead to changing velocity of a planet but unchanged speed (qualitative only) (content found only in the physics separate science qualification) (Higher tier only) lesson, made from checked teaching notes for this specification point.
Have a go first
A moon orbits a planet in a circle at constant speed. Which statement best explains why its velocity is not constant?
Listen
For a planet moving in a circular orbit, the gravitational force from the Sun acts towards the centre of the orbit. Velocity is a vector: it has both a size (speed) and a direction. Because the planet's direction of motion changes continuously around the circle, its velocity changes even though its speed stays constant. The gravitational force is always perpendicular to the planet's motion, so it changes the direction of motion rather than the speed. This is a qualitative explanation only: you need to state that the force acts towards the centre, that the direction of motion changes, and that velocity is a vector so a change of direction means a change of velocity while speed remains the same. A strong answer earns marks for these points. States that the gravitational force on the planet acts towards the centre of its circular orbit. Explains that the planet's direction of motion changes continuously as it moves around the circle. States that velocity is a vector quantity with both size and direction. Concludes that a changing direction changes the velocity while the speed stays constant. Exam tip: Define velocity as a vector before explaining why it changes.
See it
Put it in order
Put these steps in the order they happen.
- 1. Speed stays constant
- 2. Changing direction means velocity changes
- 3. Force changes the direction of motion rather than the speed
- 4. Velocity is a vector: it has size (speed) and direction
- 5. Gravitational force acts towards the centre of the circular orbit
- 6. Force is perpendicular to the planet's motion
- 7. Planet's direction of motion changes continuously around the circle
Key words
Say what each word means before you turn the card.
Circular orbits
Try answering aloud or in your head before turning the card.
Gravitational force towards the centre
Try answering aloud or in your head before turning the card.
Velocity as a vector
Try answering aloud or in your head before turning the card.
Constant speed with changing direction
Try answering aloud or in your head before turning the card.
See every definition
- Circular orbits
- A planet moves around a circle, and the gravitational force from the Sun acts towards the centre of the orbit.
- Gravitational force towards the centre
- The force on the planet acts towards the centre of its circular orbit and is always perpendicular to the planet's motion.
- Velocity as a vector
- A quantity with both a size (speed) and a direction, so a change of direction means a change of velocity.
- Constant speed with changing direction
- The planet's direction of motion changes continuously around the circle, so its velocity changes even though its speed stays constant.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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