Gravitational fields Cambridge International AS & A Level Physics revision
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In plain words
Every mass pulls on every other mass without touching it. Physicists picture this as a field: a region round a mass where another mass will feel a force. The strength of the field at a point is simply how many newtons it would pull with on each kilogram put there.
4 things to know
- A gravitational field is a region in which a mass experiences a force. Gravitational field strength g = F ÷ m: the force per unit mass, in N/kg.
- Field lines show the direction of the force on a mass. Round a planet they point inwards towards its centre (a radial field), and they are closer together where the field is stronger.
- Near the surface of the Earth the lines are almost parallel and evenly spaced. The field there is nearly uniform, with g = 9.81 N/kg.
- Gravity always attracts. It never repels.
Tips and tricks
- N/kg and m/s² are the same unit, so g is both the field strength and the acceleration of free fall.
- When you draw a radial field, put arrows on the lines pointing in towards the mass.
It lands in your notebook with its questions as flashcards.
Gravitational fields: 5 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
What is gravitational field strength?
g = F ÷ m.
What are the units of gravitational field strength?
Newtons on each kilogram.
Which way do the gravitational field lines round a planet point?
They show the direction of the force on a mass.
The gravitational field strength on the Moon is 1.6 N/kg. What is the force on a 5.0 kg mass there?
F = mg.
What does it mean when field lines are closer together?
The spacing of the lines shows the strength.
Quiz
5 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
3 questions, 5 marks. Write your answers on paper, then check them.
Gravitational fields
Cambridge International AS & A Level Physics 9702 · 5 marks · papermunch.org
Name ______________________________ Date ______________
A 3.0 kg mass feels a gravitational force of 11.1 N on the surface of Mars. Find the gravitational field strength there.[2]
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3.7 N/kg.
Describe the Earth's gravitational field far from the Earth, and close to its surface.[2]
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Far away it is radial: the field lines point towards the centre of the Earth and spread out, so the field gets weaker with distance. Close to the surface the lines are almost parallel and evenly spaced, so the field is nearly uniform.
Define gravitational field strength.[1]
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The gravitational force per unit mass acting on a small mass placed at that point.
Answers: Gravitational fields
- 1. 3.7 N/kg.
- 2. Far away it is radial: the field lines point towards the centre of the Earth and spread out, so the field gets weaker with distance. Close to the surface the lines are almost parallel and evenly spaced, so the field is nearly uniform.
- 3. The gravitational force per unit mass acting on a small mass placed at that point.