Center of Mass Motion: Velocity & Momentum

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Discussion Overview

The discussion revolves around the motion of the center of mass in a system of masses, specifically examining the relationship between the center of mass's velocity, net momentum, and total energy. The scope includes theoretical considerations and mathematical reasoning, particularly in the context of relativistic physics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant suggests that the velocity of the center of mass can be expressed as the net momentum divided by (1/c^2 times) the total energy of the system.
  • Another participant agrees with the concept but emphasizes that it holds true only in the absence of external forces.
  • A different participant notes that while the relationship is straightforward in one spatial dimension, it becomes more complex in higher dimensions and hints at a potential 4-vector approach.
  • One participant proposes using Lorentz transformations of energy and momentum to derive the velocity of the center-frame, suggesting that the linearity of the transformation applies to both single particles and sums of many particles.
  • Another participant counters that the situation is not straightforward unless the particles are non-interacting, citing the relativity of simultaneity and the need to work with the energy momentum tensor.

Areas of Agreement / Disagreement

Participants express differing views on the simplicity of the relationship between center of mass motion and relativistic effects, indicating that the discussion remains unresolved with multiple competing perspectives.

Contextual Notes

Limitations include the dependence on the assumption of non-interacting particles and the complexities introduced by the relativity of simultaneity, which are not fully resolved in the discussion.

Hiero
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If we have a system of masses in motion, will the velocity of the center of mass always be given by the net momentum divided by (1/c^2 times) the total energy of the system?
 
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Center of energy, but yes. As long as you do not subject it to external forces.
 
For motions restricted to one spatial dimension, it's easy enough to show. For more dimensions of space things get messier. I am working straight from the definitions though; maybe there is some 4vector approach?
 
Nevermind; the way I was doing it was unnecessarily difficult... We can just use the lorentz transforms of energy E and momentum P:
If we boost out of the center-frame where P=0 then the new energy will become ϒE and the new momentum will be ϒvE/c^2 and hence the velocity of the center-frame (v) will be given by c^2(P'/E')
(The linearity of the transformation means it doesn't matter if we're talking about a single particle or the sums of many.)
 
Last edited:
It is actually not that simple unless your particles are non-interacting. The reason for this is relativity of simultaneity. You generally need to work with the energy momentum tensor.
 

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