Gravitational fields are regions around a mass where another mass will experience a force due to gravity.

Gravitational field strength is defined as force per unit mass.

Formula:

The gravitational force between two objects is directly proportional to the product of their masses and inversely proportional to the square of the distance between the centres of the two objects, and is always attractive.

Formula:

Combining these two equations gives the gravitational field strength for a spherical mass, , at a distance :

We can see from the equation that the gravitational field strength from a spherical mass, such as a planet, follows the inverse square law.

Gravitational field lines show the direction of the force that a unit mass would experience at each point in the field. Because gravity is attractive, the field lines around a point mass—or a spherical mass like Earth—always point inward, toward the centre. A unit mass placed near Earth would therefore feel a force pulling it toward Earth’s centre.

-> Field lines indicate the direction a point mass would accelerate if placed in that field.

As the Earth is so large, on the surface of the Earth up until around , the field lines can be modelled as uniform.

A satellite is kept in orbit around a planet due to the centripetal force of gravity between the satellite and the planet. The satellite does not crash into the planet as the direction of the centripetal force is perpendicular to the satellite's direction of motion (at a tangent to the circle).

The radius of a satellite's orbit is the sum of its height above the Earth's surface and the Earth's radius. The height is often referred to as altitude in exam questions.

From the circular motion topic we know that angular velocity, , for a full orbit/rotation, can be calculated as:

We can use this equation to calculate the size of the resultant force on the satellite:

or we can use the equation:

is the linear speed of the satellite.

If the satellite is in geosynchronous orbit, the orbital period of the satellite is equal to the time for the Earth to rotate on its axis, . The satellite then appears to always be in the same location above the surface of the Earth. The satellite orbits in the plane of the equator and is used for things like GPS and communication networks.