5.4.1 Point and spherical masses

Firstly, it is important to understand that ALL objects with mass create a gravitational field. This field extends to infinity but gets weaker with distance from the centre of mass of the object. Any object placed in a gravitational field will experience an attractive force towards the centre of mass of the object that is creating the field (e.g for objects on Earth we call this gravitational attraction the objects weight).

Gravitational field strength
The gravitational field strength 'g' at a point within a gravitational field is defined as the gravitational force exerted per unit mass on a small object placed at that point within the field. It is a vector and always points to the centre of mass of the object creating the gravitational field. It can be calculated using the equation:

g = F/m

NOTE: F is the gravitational force and m is the mass of the object in the gravitational field. Gravitational field strength has the unit  N kg^-1. This unit is the same as m s^-1. This is because gravitational field strength at a point is the same as the acceleration of free fall of an object at that point (g = a).

Gravitational field patterns
We can map the gravitational field pattern around an object by using gravitational field line/lines of force. The lines never cross and always show the direction of the field (the direction of force on a mass at that point in a field). Field lines that are closer together indicate a stronger field. The field lines around a spherical mass form a radial field. The gravitational field strength decreases with distance from the centre of the mass (the field lines get further apart). We can model large planets as point masses with field lines converging at the centre of mass of the object because the radial fields for a spherical mass and a single point mass are very similar. If the field lines are parallel and equidistant the field is uniform. In a uniform field the gravitational field strength does not change.

Gravimetry
This isn't exactly in the spec but we've covered it so I thought it might be useful to cover on the blog:

Gravimetry is the precise measurement and study of a gravitational field. Earth's gravitational field can be mapped and minute variations detected (usually due to topography, e.g. mountains, craters from meteorites etc). Gravimetry is used in mineral prospecting as denser rock causes a higher than normal gravitational field on the Earths surface (indicating metal ores are present there). We also use this technique to search for oil as oil-bearing rocks have a lower density than the surrounding rock.

Superconducting gravimeters are the most accurate gravimeters. They use liquid helium to cool a superconducting sphere in a magnetic field. The weight of the field is balanced by the effects of the magnetic field. The electric current required to generate the magnetic field depends on the Earth's gravitational field strength at that point.

Different fields
It is important to remember that not just gravitational fields give rise to a force. We also have magnetic, and electric fields we cover in this spec!