Define gravitational field strength.

The gravitational force per unit mass acting on a small mass placed at that point, g = F/m.

What is the unit of gravitational field strength?

N kg⁻¹ (equivalent to m s⁻²).

State Newton's law of gravitation.

The force between two point masses is proportional to the product of their masses and inversely proportional to the square of their separation: F = Gm₁m₂/r².

How does the force change if the separation between two masses is doubled?

It falls to one quarter, because the force follows an inverse square law.

Why does the mass of a test object not appear in g = GM/r²?

It cancels when F = GMm/r² is divided by m, so g depends only on the source mass and distance.

From where is r measured for a spherical planet?

From the centre of the planet, so height above the surface plus the planet's radius.

What are the two gravitational forces between the Earth and the Moon an example of?

A Newton's third law pair: equal in size, opposite in direction, acting on different bodies.

What does a uniform gravitational field look like in a diagram?

Parallel, equally spaced field lines, as over a small region near the Earth's surface.

What is a neutral point between two masses?

A point where the fields are equal and opposite so the resultant field strength is zero.

Gravitational fields and Newton's law of gravitationEdexcel A-Level Physics: Flashcards

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Define a gravitational field.

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Define a gravitational field.: A region of space in which a mass experiences a force due to another mass.
Define gravitational field strength.: The gravitational force per unit mass acting on a small mass placed at that point, g = F/m.
What is the unit of gravitational field strength?: N kg⁻¹ (equivalent to m s⁻²).
State Newton's law of gravitation.: The force between two point masses is proportional to the product of their masses and inversely proportional to the square of their separation: F = Gm₁m₂/r².
What is the value of G?: 6.67×10⁻¹¹ N m² kg⁻².
How does the force change if the separation between two masses is doubled?: It falls to one quarter, because the force follows an inverse square law.
Give the equation for the field strength at distance r from a point mass M.: g = GM/r².
Why does the mass of a test object not appear in g = GM/r²?: It cancels when F = GMm/r² is divided by m, so g depends only on the source mass and distance.
From where is r measured for a spherical planet?: From the centre of the planet, so height above the surface plus the planet's radius.
What are the two gravitational forces between the Earth and the Moon an example of?: A Newton's third law pair: equal in size, opposite in direction, acting on different bodies.
What does a uniform gravitational field look like in a diagram?: Parallel, equally spaced field lines, as over a small region near the Earth's surface.
What is a neutral point between two masses?: A point where the fields are equal and opposite so the resultant field strength is zero.

Exam questions on Gravitational fields and Newton's law of gravitation

A mission planner is comparing the gravitational field of Mars with that of the Earth before designing a rover. Mars may be treated as a uniform sphere of mass 6.42×10²³ kg and radius 3.39×10⁶ m. The gravitational constant is G = 6.67×10⁻¹¹ N m² kg⁻².
A rover of mass 185 kg is on the surface of Mars. Calculate the gravitational force on the rover.2 marks
A communications satellite of mass 850 kg moves in a circular path at a distance of 4.22×10⁷ m from the centre of the Earth. The Earth may be treated as a point mass of 5.97×10²⁴ kg at its centre.
Calculate the gravitational field strength of the Earth at the position of the satellite.2 marks
A lunar lander is travelling along the straight line joining the centres of the Earth and the Moon. The mass of the Earth is 5.97×10²⁴ kg, the mass of the Moon is 7.35×10²² kg, the radius of the Moon is 1.74×10⁶ m and the distance between their centres is 3.84×10⁸ m. Take G = 6.67×10⁻¹¹ N m² kg⁻².
Show that the gravitational field strength at the surface of the Moon is about 1.6 N kg⁻¹.3 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).