How does gravity keep objects in orbit, and how does weight differ between bodies?
Orbits and gravity: how gravity provides the force for circular orbits, why orbital speed sets the orbit radius, and how weight and g differ between the Earth, Moon and other bodies.
A focused answer to Edexcel GCSE Physics 7.1 and 7.5 to 7.7 (separate physics), covering how gravity provides the centripetal force for circular orbits, why changing velocity at constant speed happens, how orbital speed relates to orbit radius, and how weight and gravitational field strength differ between bodies in space.
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What this dot point is asking
Edexcel statements 7.1 and 7.5 to 7.7 (separate physics) want you to explain how the force of gravity leads to a changing velocity at constant speed in a circular orbit, how the orbital radius must change if the orbital speed changes, the orbits of moons, planets, comets and satellites, and how weight and gravitational field strength differ between bodies.
Gravity and circular orbits
Without gravity, a planet would move in a straight line (Newton's first law). Gravity constantly pulls it towards the Sun, bending its path into a curve. The force does not speed the planet up or slow it down for a circular orbit; it only changes the direction, which is what keeps the planet circling.
Constant speed, changing velocity
This is a key examinable subtlety. Speed (a scalar) can stay constant while velocity (a vector) changes, because the direction is always changing as the object goes round. So an orbiting satellite is constantly accelerating (changing velocity) even though its speed is steady, and that acceleration is caused by gravity acting towards the centre.
Orbital speed and radius
Objects closer to the Sun must move faster to stay in a stable orbit, while distant objects move more slowly; this is why Mercury orbits much faster than Neptune. Comets, on very elongated orbits, speed up as they approach the Sun (small radius) and slow down far away (large radius). You are only expected to describe this relationship qualitatively.
Weight and g on different bodies
A larger, more massive body has a stronger gravitational field at its surface, so objects weigh more there. On the smaller-mass Moon, is about one sixth of Earth's, so an astronaut weighs about one sixth as much, even though their mass (and how much matter they contain) is unchanged.
How Edexcel examines this
These statements are separate-physics only and examined on both tiers within that route. A frequent question asks you to explain how gravity keeps a planet in a circular orbit at constant speed but changing velocity; the full-mark answer states that gravity acts towards the centre, continually changing the direction of motion, and that a changing direction means a changing velocity even though the speed is constant. The speed-radius relationship is examined qualitatively, rewarding the statement that a faster orbit has a smaller radius (and vice versa), often applied to comets speeding up near the Sun. Weight questions reuse from Topic 2, asking why weight differs between the Earth and Moon and what happens to mass; examiners reward linking the smaller weight to the smaller and stating that mass is unchanged. The most penalised errors are claiming the speed (rather than the velocity) changes in a circular orbit and saying mass changes between bodies, so guard against both.
Try this
Q1. State the direction of the gravitational force on a planet in orbit around the Sun. [1 mark]
- Cue. Towards the centre of the orbit (towards the Sun).
Q2. Explain why a planet's velocity changes even at constant speed in a circular orbit. [1 mark]
- Cue. Its direction of motion is continually changing, and velocity is a vector.
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 20213 marksExplain how gravity allows a planet to move in a circular orbit around the Sun at a constant speed but with a changing velocity.Show worked answer →
Gravity provides a force on the planet that always acts towards the centre of the orbit (towards the Sun) (1 mark). This force continually changes the direction of the planet's motion, keeping it moving in a circle (1 mark). Because velocity is a vector that includes direction, the velocity is constantly changing (the direction changes) even though the speed (the magnitude) stays the same (1 mark). Markers reward the gravitational force acting towards the centre, that it changes the direction of motion, and that a changing direction means a changing velocity at constant speed.
Edexcel 20223 marksExplain why the weight of an astronaut is different on the Moon compared with on Earth, and state what happens to the astronaut's mass.Show worked answer →
Weight depends on the gravitational field strength, which is smaller on the Moon (about ) than on Earth (about ) because the Moon has a smaller mass (1 mark). So the astronaut weighs less on the Moon, since gives a smaller weight when is smaller (1 mark). The astronaut's mass stays the same, because mass is the amount of matter and does not depend on location (1 mark). Markers reward linking the smaller weight to the smaller gravitational field strength on the Moon and stating that mass is unchanged.
Related dot points
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Sources & how we know this
- Pearson Edexcel GCSE (9-1) Physics (1PH0) specification — Pearson (2016)