# Gravitational Fields

## Gravitational Forces

A force field isa region in which an object will experience a non-contact force, such as a gravitational field. Any object with a mass will experience an attractive force if you put in the gravitational field of another object. Force fields are vectors.

The Earth's gravitational field is radial and the lines of the force meet at the centre of the Earth. The further away an object is from the Earth, the weaker the gravitational force. Close to the surface of the Earth, the field lines are (almost) uniform - parallel and equally spaced apart.

Point masses - objects which behave as if their mass is concentrated at the centre.

The Law of Gravitation is an inverse square law, meaning that if the masses increase then the force will decrease and if the distance doubles, the force will be one quarter of the original.

Gravitational Field Stregth is force per unit mass and its value depends on where you are in the field. g is a vector quantity and is always pointing towards the centre of mass whose field you are describing. It is often referred to as the acceleration due to gravity.

At some point between the Earth and the Moon, the Moon's gravity will be larger than that of the Earth. At some point the resultant field strength will be zero - this is known as the neutral point.

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## Gravitational Potential

Gravitational potential is the GPE that a unit mass at a certain point would have.

Gravitational potential is negative on the surface of the mas and increases with distance from the mass. This negative energy can be thought of as you having to do work against the gravitational field to move an object out of it.

Gravitational potential will be zero at the point infinity.

To escape a gravitational field, a mass must travel at the escape velocity. Escape velocity is the velocity at which an object's kinetic energy is equal to minus its gravitational potential energy. Therefore, the total energy is zero.

Gravitational Potential Difference is the energy needed to move a unit mass. When you move an object and do work against the force of gravity, the amount of energy you need depends on the mass of the object and the GP difference you move it through.

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## Equipotentials

Equipotentials are lines that join all points with the same potential. If you were to move along an equipotential, your gravitational potential difference would equal zero. The work done would also equal zero.

Equipotentials and field lines are perpendicular.

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## Orbits

An orbiting satellite has both KE and Potential energy and its total energy is always constant.

In a circular orbit: speed, distance above mass and energy are constant.

In an elliptical orbit: a satellite will speed up as its height decreases, meaning its KE will increase and GPE will decrease. Total energy still remains constant.

Geostationary Satellites

• Synchronous orbit - orbital period is the same as the rotational period.
• Always above the same point on earth.
• Directly above equator.
• Travels at same angular speed as the Earth turns below it.
• Orbital radius around 42 000 km
• Useful for TV and telephone signals - you don't have to alter the angle of your reciever.
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## Orbits 2

Low Orbit Satellites

• Orbit between 180-2000km above Earth.
• Cheap to launch an dhav eless powerful transmitters.
• Used for monitoring weather as they can see the Earth's surface in a high level of detail.
• Orbit usually lie in a plane that includes the north and south pole.
• Each orbit is over a new part of the Earth's surface.
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