Gravitational Fields | Edexcel A-Level Physics (9PH0)
Gravitational Fields
- 62 questions
- 6 subtopics
- Paper 2: Advanced Physics II
- Paper 2 and Paper 3
Paper 2 content on gravitation: field strength and the field of a point mass, Newton's law of universal gravitation and the orbits that follow from it, gravitational potential, and the point-by-point comparison with electric fields..
It covers gravitational fields and gravitational field strength, the field of a point mass and calculations with g, newton's law of universal gravitation, orbital speed, period and geostationary satellites, gravitational potential and potential energy and comparing gravitational and electric fields.
Sample questions from Gravitational Fields
Answer each one closed book first, then open the answer.
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Gravitational fields and gravitational field strength
State the defining equation for gravitational field strength and give its unit:
Show the answer
g = F/m, measured in N kg⁻¹. -
Gravitational fields and gravitational field strength
What do the field lines look like in the region very close to the Earth's surface?
Show the answer
They are parallel, equally spaced and directed vertically downwards, showing a uniform field. -
The field of a point mass and calculations with g
The Earth has a mass of 5.97 × 10²⁴ kg and a radius of 6.37 × 10⁶ m. What is g at its surface?
Show the answer
g = 6.67 × 10⁻¹¹ × 5.97 × 10²⁴ ÷ (6.37 × 10⁶)² = 9.81 N kg⁻¹. -
The field of a point mass and calculations with g
Two masses lie some distance apart. What happens to the resultant field strength at a point on the line between them?
Show the answer
The two fields point in opposite directions there, so they partly cancel, and at one particular point the resultant field strength is zero. -
Newton's law of universal gravitation
In the equation for universal gravitation, what exactly does r measure for two spherical bodies?
Show the answer
It is the distance between their centres, not the gap between their surfaces. -
Newton's law of universal gravitation
The separation between two masses is tripled. By what factor does the gravitational force change?
Show the answer
The force falls to one ninth of its original value. -
Orbital speed, period and geostationary satellites
Does a heavier satellite need to travel faster in a given orbit than a lighter one?
Show the answer
No, because the satellite's mass cancels from the equation, so orbital speed depends only on the central mass and the orbital radius. -
Orbital speed, period and geostationary satellites
What is the radius of an orbit around the Earth whose period is 24 hours, taking GM as 3.98 × 10¹⁴ N m² kg⁻¹?
Show the answer
r³ = GMT²/(4π²) = 3.98 × 10¹⁴ × (8.64 × 10⁴)² ÷ 39.5 = 7.53 × 10²² m³, so r = 4.2 × 10⁷ m.
The 6 subtopics
One subtopic is one session. Work down the list.
| Subtopic | What it covers | Questions |
|---|---|---|
| Gravitational fields and gravitational field strength | Recall questions on what a force field is, the definition g = F/m and its unit, why N kg⁻¹ equals m s⁻², and the field lines around a planet and near the surface. | 10 |
| The field of a point mass and calculations with g | Recall questions on the derivation of g = Gm/r², treating a sphere as a point mass, how g varies outside a planet, and finding g at a planet's surface or at orbital radius. | 10 |
| Newton's law of universal gravitation | Recall questions on the law in words and as an equation, what r measures for two spheres, why the force is always attractive, and how it changes with mass and separation. | 8 |
| Orbital speed, period and geostationary satellites | Recall questions on what supplies the centripetal force, deriving orbital speed and period, the period-radius relationship, and the conditions for a geostationary orbit. | 13 |
| Gravitational potential and potential energy | Recall questions on the definition of gravitational potential, why it is always negative, its value at infinity, equipotential surfaces, and the energy needed to escape. | 12 |
| Comparing gravitational and electric fields | Recall questions on where the two field types agree and where they differ, comparing the force laws, definitions and potentials, and why gravity governs the universe though it is far weaker. | 9 |
How the guide is worked
Answering a question from memory stores it far better than reading the answer again. The guide runs that as a fixed procedure on one subtopic at a time, about twenty minutes a session.
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Step 1 · Closed book
Cover the answers. Work through one subtopic and write down what you can. Leave blanks where you have nothing.
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Step 2 · Open book
Go back to the top. Read each printed answer and write it out in full, including the ones you had right.
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Step 3 · Closed book again
Same questions, same order, from memory. The gap between pass one and pass three is the session result.
Read the full method, the return schedule and the research behind it.
Nearby topics
Edexcel A-Level Physics Active Recall Guide
Every topic, not just this one. 1,616 questions with their answers.