• There is a force of attraction between all masses
• This force is known as the ‘force due to gravity’ or the weight
• The Earth’s gravitational field is responsible for the weight of all objects on Earth
• A gravitational field is defined as:

A region of space where a mass experiences a force due to the gravitational attraction of another mass

• The direction of the gravitational field is always towards the centre of the mass
  • Gravitational forces cannot be repulsive
• The strength of this gravitational field (g) at a point is the force (F_g) per unit mass (m) of an object at that point:

• Where:
  • g = gravitational field strength (N kg^-1)
  • F_g = force due to gravity, or weight (N)
  • m = mass (kg)
• This equations tells us:
  • On planets with a large value of g the gravitational force per unit mass is
• Mass remains the same at all points in space, however, weight will be a lot greater meaning a human will be unable to fully stand up

A person’s weight on Jupiter would be so large a human would be unable to fully stand up

Worked example: Gravitational field

• 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 you are from the centre of mass of the sphere