Unit Overview

Gravitational fields

CIE A-Level Physics· 5 min read 📊 n/a

1. Unit at a Glance

Gravitation is the fundamental force that governs large-scale motion in the universe, holding solar systems together and keeping satellites in stable orbits. This unit builds from foundational definitions to applied problem-solving, connecting abstract field concepts to real-world orbital motion.

The learning arc progresses step-by-step: you will start with basic field definitions, then learn Newton's universal gravitational law, quantify field strength and potential, and end by applying all prior concepts to solve orbital motion problems.

This unit is split into 5 connected sub-topics:

Gravitational field concepts

Introduces gravitational fields as force fields and defines core terms for describing field interactions with mass.

★★⏱ 5 min

Newton's law of gravitation

Covers Newton's universal law of gravitation and how to calculate force between point and spherical masses.

★★⏱ 6 min

Gravitational field strength

Explains how to calculate gravitational field strength for point masses, uniform spheres, and uniform near-Earth fields.

★★★⏱ 7 min

Gravitational potential

Defines gravitational potential and potential energy, and links these concepts to field strength and gravitational force.

★★★★⏱ 8 min

Orbital motion

Applies gravitational concepts to analyze circular and geostationary orbits of planets, moons, and artificial satellites.

★★★⏱ 8 min

2. Common Pitfalls

Wrong move:

Confusing gravitational field strength with gravitational potential.

Why:

Both are properties of a point in a gravitational field, but have different physical meanings and units.

Correct move:

Remember $g = - d V / d r$ : $g$ is force per unit mass, $V$ is energy per unit mass.

Wrong move:

Using positive values for gravitational potential with the standard infinity zero reference.

Why:

Zero potential is defined at infinity, and work must be done to move a mass from any point in the field to infinity.

Correct move:

Always use negative values for gravitational potential unless explicitly told to use a different reference.

Wrong move:

Adding extra force terms to centripetal force calculations for stable orbital motion.

Why:

Confusion between orbital motion and circular motion near a planet's surface where multiple forces act.

Correct move:

For stable orbits, gravitational force provides all the centripetal force required, so equate them directly.

3. Quick Reference Cheatsheet

Concept	Key Formula
Newton's law of gravitation	$F = G \frac{m _{1} m _{2}}{r ^{2}}$
Gravitational field strength (point mass)	$g = G \frac{M}{r ^{2}}$
Gravitational potential (point mass)	$V = - \frac{GM}{r}$
Gravitational potential energy	$U = - \frac{GM m}{r}$
Orbital speed	$v = \frac{GM}{r}$
Kepler's third law	$T^{2} = \frac{4 π ^{2} r ^{3}}{GM}$
Escape speed	$v_{esc} = \frac{2 GM}{r}$

What's Next

Begin your learning with the first sub-topic on core gravitational field concepts to build a solid foundation for the rest of the unit. After completing all sub-topics in this unit, you will move on to study electric fields, which share many analogous mathematical and conceptual structures with gravitational fields.