Physics Help Forum relativistic energy problem

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 Sep 7th 2008, 12:13 AM #1 Junior Member   Join Date: Apr 2008 Posts: 5 relativistic energy problem *7. In a particle accelerator, an electron with a relativistic energy of 25 GeV (as measured 1 by an observer at rest relative to the accelerator) collides head on with a proton with a relativistic energy of 800 GeV (as measured by an observer at rest relative to the accelerator). Note that 1 GeV = 109 eV. What is the total energy in the centre-of-mass frame (the frame of reference in which the total three-momentum is zero)? [10 marks] any help would be fine. i got a solution of 1.32E-7 J if anybody would like to check. if it is wrong, can you walk me through how to do it. i dont want a solution to the problem, i just want to grasp the concept thanks
Sep 7th 2008, 11:53 AM   #2

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 Originally Posted by ah-bee *7. In a particle accelerator, an electron with a relativistic energy of 25 GeV (as measured 1 by an observer at rest relative to the accelerator) collides head on with a proton with a relativistic energy of 800 GeV (as measured by an observer at rest relative to the accelerator). Note that 1 GeV = 109 eV. What is the total energy in the centre-of-mass frame (the frame of reference in which the total three-momentum is zero)? [10 marks] any help would be fine. i got a solution of 1.32E-7 J if anybody would like to check. if it is wrong, can you walk me through how to do it. i dont want a solution to the problem, i just want to grasp the concept thanks
Don't you mean "center of momentum" frame? There is a difference between this and center of mass, though from what you describe you want center of momentum.

When you calculate the momentum of each particle you will find that the lab frame itself is the COM frame. So the total energy in the COM frame is 825 GeV. (Unless you have a good reason to I'd advise leaving it like this, not in Joules.) Which you apparently already know.

-Dan
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