Gravitational Fields | OCR A-Level Physics A (H556)
Gravitational Fields
- 84 questions
- 7 subtopics
- Module 5: Newtonian world and astrophysics
- Paper 1 and Paper 3
Newton's law of gravitation and the field it describes: field strength, Kepler's laws and orbits, and gravitational potential and potential energy..
It covers gravitational fields and field strength, newton's law of gravitation, field strength of a point mass and the uniform field near Earth, kepler's laws and orbital motion, orbital period and geostationary orbits, gravitational potential and gravitational potential energy and escape velocity.
Sample questions from Gravitational Fields
Answer each one closed book first, then open the answer.
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Gravitational fields and field strength
Explain why gravitational field lines around an isolated planet are drawn pointing inward rather than outward.
Show the answer
Because a test mass placed in the field would experience a force directed towards the planet's centre, and field lines show the direction of this force -
Gravitational fields and field strength
Calculate the gravitational field strength when gravitational force = 532 N, mass = 59.5 kg.
Show the answer
Gravitational field strength = 532 ÷ 59.5 = 8.94 N kg⁻¹. -
Newton's law of gravitation
Write the equation for the gravitational force between two point masses and define each symbol.
Show the answer
F = −GMm/r², where G is the gravitational constant, M and m are the two masses, and r is the separation between their centres. -
Newton's law of gravitation
Why is the gravitational force assigned a negative value in the equation F = −GMm/r²?
Show the answer
Because gravitational force is always attractive, pulling masses together rather than pushing them apart, so it acts opposite to the direction of increasing r. -
Field strength of a point mass and the uniform field near Earth
Calculate the gravitational field strength when gravitational constant = 6.67 × 10⁻¹¹ N m² kg⁻², mass = 8.13 × 10²⁵ kg, distance from centre = 4.66 × 10⁷ m.
Show the answer
Gravitational field strength = (6.67 × 10⁻¹¹ × 8.13 × 10²⁵) ÷ (4.66 × 10⁷)² = 2.50 N kg⁻¹. -
Field strength of a point mass and the uniform field near Earth
What is the relationship between gravitational field strength and acceleration of free fall close to the Earth's surface?
Show the answer
They are numerically equal; the gravitational field strength g has the same value as the acceleration of free fall. -
Kepler's laws and orbital motion
State Kepler's third law of planetary motion.
Show the answer
The square of the orbital period of a planet is proportional to the cube of its orbital radius (T² ∝ r³). -
Kepler's laws and orbital motion
After equating gravitational and centripetal forces, how do you simplify to find an expression for v²?
Show the answer
Cancel m from both sides and simplify to obtain v² = GM/r.
The 7 subtopics
One subtopic is one session. Work down the list.
| Subtopic | What it covers | Questions |
|---|---|---|
| Gravitational fields and field strength | Recall questions on modelling a sphere as a point mass, field lines, and g as force per unit mass. | 16 |
| Newton's law of gravitation | Recall questions on the statement of the law, the inverse square relationship, and the gravitational constant. | 8 |
| Field strength of a point mass and the uniform field near Earth | Recall questions on gravitational field strength g = −GM/r² for a point mass with calculations, why the field is approximately uniform near the Earth's surface, and why g equals the acceleration of free fall. | 9 |
| Kepler's laws and orbital motion | Recall questions on the three laws and on gravity supplying the centripetal force for an orbit. | 11 |
| Orbital period and geostationary orbits | Recall questions on T² ∝ r³, and on the conditions for and uses of a geostationary orbit. | 11 |
| Gravitational potential | Recall questions on gravitational potential and why it is negative, V = −GM/r, its unit and where it applies, and finding and calculating the change in potential between two points. | 10 |
| Gravitational potential energy and escape velocity | Recall questions on gravitational potential energy E = −GMm/r and its link to potential, how it changes with distance, the area under a force-distance graph, and deriving and calculating escape velocity. | 19 |
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
OCR A-Level Physics A Active Recall Guide
Every topic, not just this one. 1,862 questions with their answers.