What is the unit of g?

N kg⁻¹ (equivalent to m s⁻²).

Is g a scalar or a vector?

A vector, in the direction of the force on a mass.

What do closer field lines mean?

A stronger gravitational field.

What do the field lines look like outside a spherical planet?

Radial, directed towards the centre of the planet.

Why does the test mass not appear in g = GM/r^2?

It cancels when F = GMm/r² is divided by m.

What distance is r in g = GM/r^2?

The distance from the centre of the mass producing the field.

How do two fields combine between two masses?

They act in opposite directions, so subtract; the resultant is zero at the neutral point.

Why do astronauts in orbit feel weightless?

They are in free fall with the same acceleration as the spacecraft, so no contact force acts on them.

Gravitational field strengthAQA A-Level Physics: Flashcards

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Define gravitational field strength.

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Define gravitational field strength.: The gravitational force per unit mass acting on a small test mass at that point, g = F/m.
What is the unit of g?: N kg⁻¹ (equivalent to m s⁻²).
Is g a scalar or a vector?: A vector, in the direction of the force on a mass.
What do closer field lines mean?: A stronger gravitational field.
What do the field lines look like outside a spherical planet?: Radial, directed towards the centre of the planet.
What do field lines look like in a uniform field?: Parallel and equally spaced.
State the equation for g in a radial field.: $g = GM/r^2$ .
Why does the test mass not appear in $g = GM/r^2$ ?: It cancels when F = GMm/r² is divided by m.
How does g change if r is doubled?: It falls to one quarter.
What distance is r in $g = GM/r^2$ ?: The distance from the centre of the mass producing the field.
How do two fields combine between two masses?: They act in opposite directions, so subtract; the resultant is zero at the neutral point.
Why do astronauts in orbit feel weightless?: They are in free fall with the same acceleration as the spacecraft, so no contact force acts on them.

Exam questions on Gravitational field strength

A Mars lander team models Mars as a uniform sphere of mass 6.42 × 10²³ kg and radius 3.39 × 10⁶ m. A small test mass is released at the surface of the planet.
Calculate the gravitational field strength at the surface of Mars. Use $G = 6.67 \times 10^{-11}\ \mathrm{N\,m^2\,kg^{-2}}$ .2 marks
The gravitational field around an isolated, uniform planet is represented by gravitational field lines drawn in the space outside the planet.
Explain why the gravitational field near the surface of the planet can be treated as uniform over a height of a few metres, but not over a height of several thousand kilometres.2 marks
The International Space Station orbits the Earth at a height of 400 km above the surface. The Earth may be treated as a uniform sphere of mass 5.97 × 10²⁴ kg and radius 6.37 × 10⁶ m.
Calculate the gravitational field strength at the height of the space station. Use $G = 6.67 \times 10^{-11}\ \mathrm{N\,m^2\,kg^{-2}}$ .3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).