P2 - Ch21 - Gravitational Fields

Gravitational field

The force field around a mass is a gravitational field. A force field is a region where a body will experience a non contact force.

Gravitational field line

The direction, and relative magnitude, of force on a small mass placed in the gravitational field of a large mass.

Gravitational field strength

The gravitational field strength at a point is the force per unit mass on a small mass at that point. (The gravitational field strength is equal to the negative of the potential gradient.)

Newton's Law of Gravitation

The force of attraction between two masses is directly proportional to the product of the two masses and inversely proportional to the square of their separation.

Gravitational potential energy

The energy of an object due to its position in a gravitational field (position for zero g.p.e. is at infinity).

Gravitational potential

The work done per unit mass in moving a small object from infinity to that point (i.e. the gravitational potential energy per unit mass).

Equipotential

A 2-dimensional surface of constant potential. No work needs to be done to move along an equipotential surface.

Potential gradient

The potential gradient at a point in a gravitational field is the change of potential per metre at that point.

Uniform field

Gravitational field strength is the same everywhere. Field lines are parallel and equally spaced.

Radial field

Due to a point mass or uniform spherical mass. Gravitational field strength follows inverse square law.

Escape velocity

The minimum velocity an object must be given to escape from a large mass (e.g. planet) when projected vertically from the surface.

Geostationary satellite

A satellite which orbits the Earth directly above the equator and has a time period of exactly 24 hours. (Note that a geosynchronous orbit is a 24 hour orbit inclined to the equator.)