# AQA A-Level Physics/Gravitational Fields

Newton’s Law- Gravity is a universal attractive force acting between all matter. Doubling the mass of an object doubles the force between the two objects whereas doubling the distance between to two objects makes the force ¼ of the force before, F is inversely proportional to the distance squared.

F is the force in Newtons between the two objects

G is the universal gravitational constant and m1 and m2 are two masses in kg, whose centres are r distance apart, measured in metres.

Gravitational Field Strength: The gravitational field strength, g, is the force per unit mass (measured in Newtons per kilogram). The magnitude of g in a parallel field is found by using this equation.

The magnitude of g in a radial field however is given by this equation.

Gravitational Potential: The gravitational potential at a point in a field is the potential energy per unit mass ( so for 1 kg). It is the work done by bringing a unit mass from infinity to that point within the field. The gravitational potential at an infinite distance is zero. Gravitational potential is scalar.

The work done W (measured in Joules) in moving mass m from a point in a field with gravitational potential V1 to gravitational potential V2 is given by the equation:

Orbits of planets and satellites: The relationship between the orbital speed and the radius of a planet can be applied to the orbit of a satellite around the Earth by replacing the mass of the Sun with that of the Earth. This enables the speed of the satellite to be calculated at any orbital radius. Some communications satellites occupy geo-synchronous orbits. A satellite in geo-synchronous orbit:

-Orbits above the equator

-Remains in the same position relative to the Earth’s surface

-Has an orbit time (or period) of 24 hours