Representing Gravitational Fields (AQA A Level Physics): Revision Note

Gravitational field lines

• The strength and direction of a gravitational field can be represented by gravitational field lines

  • The gravitational field lines around a point mass are radially inwards

  • The gravitational field lines of a uniform field, where the field strength is the same at all points, are represented by equally spaced parallel lines, for example, the field lines near the Earth’s surface

• Radial fields are considered to be non-uniform fields

  • The gravitational field strength $g$ is different depending on how far a point is from the centre

• Parallel field lines on the Earth’s surface are considered a uniform field

  • The gravitational field strength $g$ is the same throughout

Point mass approximation

• For a point outside a uniform sphere, the mass of the sphere may be considered to be a point mass at its centre

  • A uniform sphere is one where its mass is distributed evenly

• The gravitational field lines around a uniform sphere are therefore identical to those around a point mass

• An object can be regarded as a point mass when:

It has an even mass distribution and the distance being considered is larger than its size

• For example, when drawing field lines around planets, the lines are drawn to converge at the centre

• Radial fields are considered non-uniform fields

  • So, the gravitational field strength $g$ is different depending on how far an object is from the centre of mass of the sphere