Topic 12: Gravitational Fields — Edexcel A-Level Physics
Test yourself on Topic 12: Gravitational Fields with PEARSON EDEXCEL A-Level practice questions.
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Topic 12: Gravitational Fields explained
A gravitational field is a force field: a region of space in which a mass placed there experiences a gravitational force.
Read the full explanation
The field is created by a source mass, and any other mass within the region feels a force directed towards the source. The idea of a field lets us describe action at a distance without needing contact. Field strength g at a point is the force per unit mass, g = F/m, so it measures how strong the field is. Near the Earth's surface g is approximately uniform and points downwards; around a point mass the field is radial and its strength falls with distance. Recognising that the field exists because a mass experiences a force is the key definition to apply when identifying whether a region is a gravitational field.
Your focus
- State the definition of a gravitational field as a region where a mass experiences a force.
- Explain that gravitational field strength is the force per unit mass, g = F/m.
- Distinguish between uniform and radial gravitational fields and describe their field lines.
Topic 12: Gravitational Fields exam tips
Marking Points
- Defines a gravitational field as a region (of space) in which a mass experiences a gravitational force.
- Identifies that the field is a force field produced by a source mass and acts on any other mass placed in it.
- States that gravitational field strength is force per unit mass, g = F/m, and is a vector directed towards the source mass.
- Distinguishes a uniform field (parallel field lines, constant g) from a radial field (field lines towards a point mass, g decreasing with distance).
Examiner Tips
- 💡Use the exact phrase 'region where a mass experiences a force' when defining a gravitational field.
- 💡When asked about field strength, quote g = F/m and give the unit N kg⁻¹.
- 💡Sketch field lines to show direction: radial lines point towards the source mass; uniform lines are parallel and equally spaced.
Common Mistakes
- Describing a gravitational field as a region containing mass rather than a region where a mass experiences a force: the defining feature is the force on a test mass.
- Treating field strength as a force rather than force per unit mass: g = F/m has units N kg⁻¹.
- Assuming g is always constant: it is only approximately uniform near a planet's surface and decreases with distance in a radial field.