Gravitational Fields

AQA A level Physics

Gravitational Field

A region surrounding a mass in which any other object with mass will experience an attractive force

Gravitational Field Strength

The force per unit mass exerted on a small test mass placed within the field

Gravitational Potential

The work done per unit mass required to move a small test mass from infinity to that point

Gravitational Potential Energy

The component of an object’s energy due to its position in a gravitational field

Kepler’s Third Law

The square of an object’s orbital period is directly proportional to the cube of its orbital radius

T² α r³

Field Line

A line representing the path that a mass would take if placed in the gravitational field

Equipotential

A surface of constant potential at right angles to radial field lines.

(constant potential = no work needs to be done to move along the surface)

Radial Field

Field lines end at the centre of mass and tail back to infinity. Become more spread out the further from c.o.m

Uniform Field

Field lines are parallel and equally spaced. Field strength is equal in all areas of the field.

Can assume parallel field lines at surface of Earth

Escape Velocity

The minimum velocity required by an object to be able to escape a gravitational field of a mass when projected vertically from its surface

Newtons’s law of gravitation

Magnitude of the gravitational force between two masses is directly proportional to the product of the masses, and is inversely proportional to the square of the distance between them

(where the distance is measured between the two centres of the masses)

Deducing ΔV from graph of g on r

Area under the graph

Deducing g from graph of ΔV on r

Gradient

Synchronous Orbit

Period of the orbit = rotational period of object it is orbiting

Geostationary Satellite

A satellite that orbits above the equator with a 24hr period, so it will always remain at the same position above Earth. Orbit approx. 36,000km above Earth

Low orbit satellites

Significantly lower orbit than geostationary. Therefore travel much faster so orbital period shorter.

Therefore require less powerful transmitters and potentially orbit across entire Earth’s surface

Applications of low orbit satellites

Communication but since can travel quickly, many satellitles must work together to allow constant coverage for a region

Monitoring weather, making scientific observations about places which are unreachable, and military applications

Total energy of a satellite

kinetic energy + potential energy