# Gravitational Felds

1.6.2 Gravitational elds
Surrounding every mass is a gravitational eld, if another mass enters the eld it experiences a force due to gravity. Note: We consider the mass of a spherical body to be concentrated at it’s centre when looking at the eld outside the body.

Gravitational elds theoretically stretch out to in nity in all directions (and theoretically, all potential dimensions…), so if you are in a gravitational eld, you have weight due to the eld. So there is gravity in space due to the Earth and other planets, and depending on the distance from the Earth, the strength can vary. As a result, ‘weightlessness’ cannot truly exist, but it can appear to occur when a mass is between two large masses since their gravitational forces’ can cancel out.
1.6.3 Gravitational potential
Gravitational potential is the amount of work per unit mass that a body would have to have done to it to move it from in nity to a point in the eld. At in nity, the gravitational potential is zero, but to move towards in nity, you must have work done to you, so if you add energy to a body and it’s nal energy state is zero, potential must be negative.

The orbital speed of matter beyond the visible section of a galaxy (i.e. in the non-visible, but still detectable section, since we use electromagnetic radiation and Doppler e ect to measure orbital speed) spins faster than expected for the amount of mass which we believe to be concentrated there.
Dark matter Matter which we can’t see, or detect by any sort of radiation, but whose existence we infer from its gravitational e ects.
Radial velocity of a star [in the context of Doppler shift] This is the component of a star’s velocity along the line joining it and an observer on the Earth.