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

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 an object is from the centre of mass of the sphere

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

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