CIE A Level Physics (9702) 2019-2021

Revision Notes

15.1.1 Gravitational Fields

Defining Gravitational Field

  • 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 (Fg) per unit mass (m) of an object at that point:

Defining Gravitational Field equation 1

  • Where:
    • g = gravitational field strength (N kg-1)
    • Fg = 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

gravitational field strength, downloadable AS & A Level Physics revision notes

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

Worked example: Gravitational field

Worked example - gravitational field, downloadable AS & A Level Physics revision notes

Exam Tip

There is a big difference between g and G (sometimes referred to as ‘little g’ and ‘big G’ respectively), g is the gravitational field strength and G is Newton’s gravitational constant. Make sure not to use these interchangeably!

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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