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
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