I suspect sptfung is referring to the idea of the equivalent principle in general relavitity. The basic idea is that gravitational force is a pseudo force, it is a force we seem to be experiencing becasue we often fail to appreciate that space-time is curved around a massive object (geodesic) and we explain the change of direction/velocity by using a force.Wah!! 寫:唔明 + 唔識sptfung 寫:I just remember that, you CANNOT DEFINE gravitational potential in general relativity.
The following is a very simplified version.
Newtonian: the Earth moves in a circular orbit because of the acceleration of the Sun's gravitational force.
General Relativity: the Earth moves in a straight line in the time-space (curved) around the sun.
When the is no "real" force, there is no work-done and therefore potential energy cannot be defined.
The following may be an introductory reading: http://en.wikipedia.org/wiki/Equivalence_principle
