AQA A Level Physics

Revision Notes

7.1.3 Representing Gravitational Fields

Gravitational Field Lines

  • The direction of a gravitational field is 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, is represented by equally spaced parallel lines
    • For example, the fields lines on the Earth’s surface

Gravitational field lines, downloadable AS & A Level Physics revision notes

Gravitational field lines for a point mass and a uniform gravitational field

  • Radial fields are considered non-uniform fields
    • The gravitational field strength g is different depending on how far you are 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

Exam Tip

Always label the arrows on the field lines! Gravitational forces are attractive only. Remember:

  • For a radial field: it is towards the centre of the sphere or point charge
  • For a uniform field: towards the surface of the object e.g. Earth

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 point mass when:

A body covers a very large distance as compared to its size, so, to study its motion, its size or dimensions can be neglected

  • An example of this is field lines around planets

Point mass sphere field lines, downloadable AS & A Level Physics revision notes

Gravitational field lines around a uniform sphere are identical to those on a point mass

 

  • Radial fields are considered non-uniform fields
    • So, the gravitational field strength g is different depending on how far you are from the centre of mass of the sphere
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