I am trying to self study this stuff.
I do not understand the difference between
$$E = \gamma m c^2$$
and
$$E^2 = (pc)^2 + (mc^2)^2$$
Are these the same thing, how do I know when to use which?
On problem 20 on test 8677, grephysics.net says to use the first one. But when I try to use the second one, I keep getting 0.6c, which is wrong and I do not understand why. Thanks
relativity energy

 Posts: 7
 Joined: Thu May 13, 2010 8:29 pm
Re: relativity energy
Yes they are the same thing. From the second one if you substitute $$p=\gamma m v$$ you recover the first equation.
Re: relativity energy
redacted.
Last edited by Skullgrid on Fri Jan 18, 2013 8:41 pm, edited 1 time in total.
Re: relativity energy
It seems like a lot of the relativity problems that I have run into on GRE stuff are simply time dilation/length contraction. Is it worth practicing and memorizing the full Lorentz transformation equations? I always have trouble with determining the order of events and remembering the time transformation equation but dont want to cram anymore in than I have to at this point

 Posts: 381
 Joined: Mon May 24, 2010 11:34 pm
Re: relativity energy
Time dilation and length contraction both use $$\sqrt{1v^2/c^2}$$ which is a little less than unity for small velocities. Just remember to multiply or divide by it in such a way that time gets increased (dilated) while length gets decreased (contracted).
Re: relativity energy
yeah, youre right. Iv got that. But im talking about applying transformation to events using
$$t' = \gamma(t\frac{v}{c^2}x)$$
I havent run into those types of problems (or velocity addition problems) on GRE stuff yet
$$t' = \gamma(t\frac{v}{c^2}x)$$
I havent run into those types of problems (or velocity addition problems) on GRE stuff yet
 HappyQuark
 Posts: 762
 Joined: Thu Apr 16, 2009 2:08 am
Re: relativity energy
Here is an example of velocity addition and time dilation on PGRE 9277ryan6 wrote:yeah, youre right. Iv got that. But im talking about applying transformation to events using
$$t' = \gamma(t\frac{v}{c^2}x)$$
I havent run into those types of problems (or velocity addition problems) on GRE stuff yet
http://physicsgrad.com/pgre/967737/
http://physicsgrad.com/pgre/927738/