Revision

Objectives

Lesson outcomes

Retrieve the key definitions, equations, and field diagrams across gravitational fields.
Practise mixed questions involving field strength, Newton's law of gravitation, orbits, and potential.
Identify which B1 gravitational fields skills need targeted revision before the test.

Syllabus

CIE 9702 syllabus points

12 linked

13.1.1 understand that a gravitational field is an example of a field of force and define gravitational field as force per unit mass
13.1.2 represent a gravitational field by means of field lines
13.2.1 understand that, for a point outside a uniform sphere, the mass of the sphere may be considered to be a point mass at its centre
13.2.2 recall and use Newton’s law of gravitation F = Gm1m2 / r2 for the force between two point masses
13.2.3 analyse circular orbits in gravitational fields by relating the gravitational force to the centripetal acceleration it causes
13.2.4 understand that a satellite in a geostationary orbit remains at the same point above the Earth’s surface, with an orbital period of 24 hours, orbiting from west to east, directly above the Equator
13.3.1 derive, from Newton’s law of gravitation and the definition of gravitational field, the equation g = GM / r 2 for the gravitational field strength due to a point mass
13.3.2 recall and use g = GM / r 2
13.3.3 understand why g is approximately constant for small changes in height near the Earth’s surface
13.4.1 define gravitational potential at a point as the work done per unit mass in bringing a small test mass from infinity to the point
13.4.2 use ϕ = –GM / r for the gravitational potential in the field due to a point mass
13.4.3 understand how the concept of gravitational potential leads to the gravitational potential energy of two point masses and use EP = –GMm / r

Definitions

Required definitions

Gravitational field
a region where a mass experiences a force; gravitational field strength is force per unit mass.
Gravitational potential
the work done per unit mass in bringing a small test mass from infinity to the point.

Lesson Notes

Student guidance and lesson notes

Overview

This lesson brings the B1 gravitational fields topic together before assessment. You should be able to move between force, field strength, circular orbit, and potential models, and choose the right approach for each question.

What You Need to Know

Field questions require clear definitions and accurate field-line diagrams.
Newton’s law of gravitation and field strength questions depend on using centre-to-centre separation.
Circular orbit questions require gravity to provide centripetal force.
Geostationary orbit explanations must include period, direction, equatorial plane, and fixed position above Earth.
Potential questions depend on the infinity reference point and careful treatment of negative values.

How to Work Through It

Start with quick retrieval of definitions, diagrams, and equations from chapter 13.
Work through mixed past-paper questions without sorting them by lesson first.
Mark each response for model choice, radius/separation, units, signs, and explanation quality.
Finish by writing a short target list for the topic test.

Check Your Understanding

Can you choose between g = F / m, F = GMm / r^2, g = GM / r^2, orbit equations, potential, and potential energy?
Can you explain why g changes with distance but is approximately constant near Earth’s surface?
Can you identify when to use altitude and when to use orbital radius?
Can you explain the negative sign in gravitational potential?

Common Mistakes

Revising equations without practising which physical model each one represents.
Measuring r from the wrong point.
Forgetting the negative sign in potential and potential energy questions.
Treating geostationary as only meaning “same period as Earth” without the other conditions.