Orbital Motion and Gravity

This lesson covers orbital motion, gravitational force, and how gravity keeps planets, moons, and satellites in orbit, as required by the AQA GCSE Physics specification (4.8.1). This is a Physics-only topic. You need to understand how gravity provides the centripetal force for orbital motion, how orbital speed and radius are related, and the difference between circular and elliptical orbits.

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Gravity and Gravitational Fields

Gravity is a non-contact force of attraction between any two objects that have mass. Every object with mass exerts a gravitational pull on every other object with mass. The strength of the gravitational force depends on:

1. The masses of the two objects — greater masses produce a stronger gravitational force.
2. The distance between the centres of the two objects — the force decreases as the distance increases.

A gravitational field is the region around a mass where another mass would experience a gravitational force. The gravitational field of a planet or star extends to infinity, but it becomes weaker with distance.

Gravitational Field Strength

The gravitational field strength (g) at a point is the force per unit mass acting on an object at that point: