5.7.1 Gravitational Fields

• Generally, the idea of a force field is any region of space in which a specific type of object will experience a force
• For example:
  • Electric fields are regions in which any object with charge experiences an electric force
  • Magnetic fields are regions in which any magnet experiences a magnetic force

• Gravitational fields are a special type of field in which any object with mass experiences a gravitational force

• Gravitational fields are set up around any object with mass
  • These fields affect any other objects with mass in their vicinity

• The Sun, for example, creates a gravitational field around it
  • The Earth, which has mass, experiences the gravitational force due to the Sun
  • This gravitational force keeps the Earth in orbit around the Sun

• Additional effects of the Moon and Sun's gravitational fields can be seen on Earth, such as the cause of tides

Direction of a Gravitational Field

• The direction of a gravitational field can be represented as a vector, the direction of which must be determined by inspection
• The direction of the vector shows the direction of the gravitational force that would be exerted on a mass if it was placed at that position in the field
• These vectors are known as field lines (or 'lines of force'), which are represented by arrows
  • Therefore, gravitational field lines also show the direction of acceleration of a mass placed in the field
• Gravitational field lines are therefore directed toward the centre of mass of a body
  • This is because the gravitational force is attractive
  • Therefore, masses always attract each other via the gravitational force
• The gravitational field around a point mass will be radial in shape and the field lines will always point towards the centre of mass

The direction of the gravitational field is shown by the vector field lines

• 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

Exam Tip