# 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:

• 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

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

#### 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

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

### Author: Katie

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.
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