The Open Door Web Site  
Mechanics Energy Changes in a Gravitational Field A mass placed in a gravitational field experiences a force. If no other force acts, the total energy will remain constant but energy might be converted from g.p.e. to k.e. Energy Possessed by a Satellite If the mass of the planet is M and the radius of the orbit of the satellite is r, then it can easily be shown that the speed of the satellite, v, is given by
therefore, if r decreases, v must increase. If the mass of the satellite is m, then the kinetic energy, K, possessed by the satellite is given by
and the potential energy, P, possessed by the satellite is given by
These equations show
Therefore, to fall from one orbit to a lower orbit, the total energy must decrease. In other words, some work must be done to decrease the energy of the satellite if it is to fall to a lower orbit. The work done, w, is equal to the change in
the total energy of the satellite,
This work results in a conversion of energy from gravitational potential energy to internal energy of the satellite (it makes it hot!). Air resistance can thus reduce the speed of the satellite along its orbit. This allows the satellite to fall towards the planet. As it falls, it gains speed. So, if a viscous drag (air resistance) acts on a satellite, it will
In principle, the satellite could settle in a lower, faster orbit but in practice it will usually be falling to a region where the drag is greater. It will therefore continue to move towards the planet in a spiral path.

