Edexcel A Level Physics
Edexcel A Level Physics Topic 12: Gravitational Fields Exam Questions
Free Edexcel A Level Physics exam questions for Topic 12: Gravitational Fields, with a mark scheme for every pack. Covers gravitational field strength, Newton's law of gravitation, gravitational potential, comparing gravitational and electric fields, and orbits.
Topic 12: Gravitational Fields exam questions
1 question pack of real past exam questions, each with its mark scheme. Do not stop after a few: answer as many as you can without notes, then mark every answer with the mark scheme. The more questions you do, the more quickly you will spot how examiners ask about each idea.
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About Edexcel A Level Physics Topic 12: Gravitational Fields
Topic 12: Gravitational Fields in Edexcel A Level Physics uses Newton's law of gravitation to describe the field around a mass. You calculate field strength g = GM/r^2 and gravitational potential V = -GM/r, interpret field lines and equipotentials, and compare gravitational and electric fields, which were met in Topic 7. Combining gravitation with circular motion from Topic 6 lets you analyse orbits of planets and satellites. It is assessed in Paper 2: Advanced Physics II. Examiners often ask you to derive orbital speed or period, explain why potential is negative and identify similarities and differences between field types.
What you need to know
- Field strength g = F/m; for a point or spherical mass, g = GM/r^2 with G = 6.67 × 10^-11 N m^2 kg^-2. Near the Earth's surface the field is approximately uniform.
- Newton's law F = Gm1m2/r^2, where r is the distance between centres. Gravitational forces are always attractive.
- Gravitational potential Vgrav = -GM/r, the work done per unit mass in moving a small mass from infinity to that point. Equipotentials are perpendicular to field lines.
- Comparing fields Both gravitational and electric radial fields follow inverse square laws and have potentials proportional to 1/r. Gravity only attracts and acts on mass; electric forces attract or repel and act on charge.
- Orbits Gravitational force provides the centripetal force: GMm/r^2 = mv^2/r, giving v = √(GM/r) and T^2 proportional to r^3. Geostationary satellites have a 24 hour period above the equator.
Common mistakes in Topic 12: Gravitational Fields
- Using height above the surface instead of distance from the centre for r.
- Giving gravitational potential a positive value.
- Saying the mass of a satellite affects its orbital speed. It cancels; speed depends only on M and r.
- Stating that gravitational and electric fields are identical. Gravitational fields only attract, and the force is much weaker for everyday particles.
- Using g = 9.81 N kg^-1 at orbital heights where the field is weaker.
Try three Topic 12: Gravitational Fields exam-style questions
1. Calculate the orbital speed and period of a satellite in circular orbit of radius 7.0 × 10^6 m around the Earth. Mass of Earth = 5.98 × 10^24 kg. (4 marks)
Show the mark scheme answer
v = √(GM/r) = √(6.67 × 10^-11 × 5.98 × 10^24/7.0 × 10^6); v = 7.5 × 10^3 m s^-1; T = 2πr/v = 2π × 7.0 × 10^6/7.5 × 10^3; T = 5.8 × 10^3 s (about 97 minutes).
2. Calculate the gravitational potential at the Earth's surface. Radius of Earth = 6.37 × 10^6 m. (2 marks)
Show the mark scheme answer
V = -GM/r = -6.67 × 10^-11 × 5.98 × 10^24/6.37 × 10^6; V = -6.3 × 10^7 J kg^-1.
3. State one similarity and two differences between gravitational and electric fields due to point sources. (3 marks)
Show the mark scheme answer
Similarity: both follow an inverse square law for force and field strength (or potential proportional to 1/r); difference: gravitational forces are always attractive whereas electric forces can be attractive or repulsive; gravitational fields act on mass whereas electric fields act on charge.
Topic 12: Gravitational Fields FAQs
What does Edexcel A Level Physics Topic 12: Gravitational Fields cover?
It covers gravitational field strength, Newton's law of gravitation, gravitational potential, comparisons with electric fields and satellite and planetary orbits.
Which paper is Topic 12 examined in?
It is assessed in Paper 2: Advanced Physics II, and like every topic it can also appear in Paper 3: General and Practical Principles in Physics.
What derivation should I practise for Gravitational Fields?
Practise equating GMm/r^2 to mv^2/r to find orbital speed and to show that T^2 is proportional to r^3.
