MadSci Network: Physics |
There is a difference between an accelerating frame of reference and an accelerating *object*. We can measure the motion of an object in an inertial frame of reference; in that inertial frame we may measure the object's velocity changing, which means that it is accelerating. This is very different from sitting on the accelerating object and trying to measure what is going on - both Newtonian mechanics and special relativity do not work without modification in this case (this is why even classically we talk about "pseudoforces" such as the centripetal force and the Coriolis force, which are artifacts due to one's being in an accelerating frame of reference, rather than true forces).Force and acceleration are derived quantities; they depend on measurements of time, distance, and mass, which relativistically will differ even in inertial reference frames. Therefore, to derive equations of motion which are valid in all inertial reference frames, we must take relativistic effects on these measurements into account. This is the source of the relativistic expression for force.
For more information, see the Special Relativity section of the Usenet Physics FAQ at http://math.ucr.edu/home/baez/physics/.
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