Gravitational Fields — A-Level Physics Revision

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What is Gravitational Fields?

This topic extends the concept of gravity from a simple force to the idea of a field. It introduces Newton's Law of Universal Gravitation, which describes the force between any two point masses. You will learn to calculate gravitational field strength (g), gravitational potential, and escape velocity, and to analyse the motion of satellites in circular orbits.

Board notes: Gravitational fields are a major A-Level topic for all exam boards (AQA, Edexcel, OCR). All boards cover Newton's law, field strength, potential, and orbital mechanics. The mathematical treatment of potential and the derivation of orbital period equations are common across all specifications.

Worked example

Common mistakes

• 1Confusing gravitational potential with gravitational potential energy. Gravitational potential is the work done per unit mass to bring a mass from infinity to a point in the field (a property of the field itself), while potential energy is the energy a specific mass has due to its position in that field.
• 2Forgetting that gravitational force and potential are always attractive. This means the force is always directed towards the mass creating the field, and the potential is always negative, with a value of zero at an infinite distance.
• 3Using the simple 'g=9.81 N/kg' in orbital mechanics. This value is only valid near the Earth's surface. For satellites and planets, the gravitational field strength must be calculated using g = GM/r², where r is the distance from the centre of the large mass.

Gravitational Fields exam questions

Exam-style questions for Gravitational Fields with mark-scheme style solutions and timing practice. Aligned to AQA, Edexcel and OCR specifications.

Frequently asked questions

• What is a gravitational field?

  A gravitational field is a region of space where a mass will experience a force. It is a vector field, meaning it has both a magnitude (field strength) and a direction at every point.

• What is escape velocity?

  Escape velocity is the minimum speed an object must have to escape the gravitational pull of a celestial body, like a planet or star, without any further propulsion. It depends on the mass and radius of the celestial body.