Gravitational Fields

AQA A - Gravitational Fields

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  • Created on: 03-01-13 12:58
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Gravitational Fields
A gravitational field exists around any mass, no matter how large or small. Other masses in this region
will feel a force, which is always attractive: a repulsive gravitational force has never been identified.
Gravitational field lines
Gravitational fields are always drawn using gravitational field lines, which show the direction of the
force on a mass placed at any point in the field. Around a spherical mass the lines look like this:
Near the Earth's surface, the field is nearly uniform and the lines are evenly spaced:
Gravitational field strength, g
The gravitational field strength g at a point in a field is the force per unit mass on an object placed at
that point:
g = m
It follows from Newton's universal law of gravitation that, around a spherical mass M:
g = GM

Where r is the distance from the centre of the mass.
Note that:

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g has units Nkg¯¹
g may also be regarded as the acceleration due to gravity (ms¯²) at the point
g is a vector quantity
the variation of g with distance from a mass is another inverse-square relationship
Earth's gravitational field
It's strength is 9.81Nkg¯¹ at the surface of the Earth
It falls off with distance above and below the surface.
Gravitational potential, V
When a mass is moved against a gravitational force, work is done.…read more

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If a mass at the Earths surface can be given a kinetic energy equal to its gravitational potential energy
(mV), It will escape completely from the Earth's gravitational field. The velocity required to do this is
called the escape velocity, it is not dependent on the mass and is given by:
r = (2gr)
Equipotential surfaces
Points that are at the same potential lie on equipotential surfaces. Around a spherical mass, these are
concentric spheres ­ or circles in two dimensions.…read more


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