# Orbits and the wider universe

> Using gravitational fields to explain satellite orbits, then connecting redshift, the Hubble law and standard-candle evidence to an expanding universe.

Canonical: https://revise-gray.vercel.app/library?topic=wjec-alevel-physics.orbits-universe
Last updated: 2026-08-01

Flagship subject: topics and specification statements are authored to the WJEC specification. No question in it has been through human review yet.

- Subject: [WJEC · A Level · Physics](https://revise-gray.vercel.app/library?subject=wjec-alevel-physics)
- Specification reference: Unit 4.3
- Difficulty: 5/5
- Assessment objectives: AO1, AO2, AO3
- Status: authored · checked · 2026-08-01

## Specification coverage

| Reference | Statement | AOs |
| --- | --- | --- |
| Unit 4.3(a) | derive and apply circular-orbit speed and period from gravitational force | AO2 |
| Unit 4.3(b) | explain Kepler relationships and conditions for a geostationary satellite | AO1, AO2 |
| Unit 4.3(c) | use redshift and the Hubble law as evidence for an expanding universe | AO2, AO3 |
| Unit 4.3(d) | evaluate uncertainty and model limitations in astronomical measurements | AO2, AO3 |

## Summary

Using gravitational fields to explain satellite orbits, then connecting redshift, the Hubble law and standard-candle evidence to an expanding universe.

## What earns marks

- For a circular orbit gravity supplies the centripetal force, so GMm/r² = mv²/r and v = √(GM/r); a larger orbit has a lower orbital speed.
- Kepler's third law follows from the same model: T² = 4π²r³/(GM), so period increases strongly with orbital radius and a synchronous orbit has a chosen period matching the rotation of the body.
- A geostationary satellite must be above the equator, move west to east in a circular orbit and have a 24-hour period; these constraints make it appear fixed to an observer on Earth.
- Redshift of distant galaxies and the Hubble relationship v = H₀d provide evidence for expansion, but distance calibration, peculiar velocities and uncertainty must be considered.

## Where marks are lost

- Using the radius above the surface instead of distance from the centre of the attracting body.
- Saying a satellite needs a continuous tangential force to keep moving in orbit; gravity changes its direction of velocity.
- Treating redshift alone as a direct measurement of distance without explaining the Hubble-law model and its uncertainties.

## Related

- [WJEC · A Level · Physics](https://revise-gray.vercel.app/library?subject=wjec-alevel-physics)
- [Orbits and the wider universe lesson](https://revise-gray.vercel.app/lesson?subject=wjec-alevel-physics&topic=wjec-alevel-physics.orbits-universe)