Orbits and Gravity

This lesson covers the role of gravity in orbital motion — as required by the Edexcel GCSE Physics specification (1PH0), Topic 7: Astronomy. This is a Paper 2 topic. You need to understand how gravity keeps planets and moons in orbit, how orbital speed varies with distance, and the difference between geostationary and polar satellites.

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Gravity and Orbits

What Keeps Objects in Orbit?

Gravity provides the centripetal force needed to keep objects moving in curved paths (orbits). Without gravity, an orbiting object would fly off in a straight line (Newton's first law — an object in motion continues in a straight line unless acted on by a force).

Key principles:

• The Sun's gravitational field keeps the planets in orbit around it.
• A planet's gravitational field keeps its moons in orbit around it.
• The Earth's gravitational field keeps artificial satellites and the Moon in orbit.
• Gravity acts as a centripetal force — it constantly pulls the orbiting object towards the centre (the more massive body), preventing it from flying off in a straight line.