AQA A Level Physics复习笔记7.1.3 Representing Gravitational Fields

• 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 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

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