# 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 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

#### 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

### Author: Ashika

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.
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