Homework Help: Relativistic momentum

1. Jan 29, 2008

supercali

[SOLVED] relativistic momentum

1. The problem statement, all variables and given/known data
a particle with mass M and v=0 decomposes to a massless particle and another particle with the mass Q.
find the momentum of the Q particle

2. Relevant equations
$$E^2_{cm}=m^2c^4=(\sum{E})^2-(\sum{P})^2c^2$$

3. The attempt at a solution
i tried using that equation and also tried to do some equations with the conservation of energy and momentum but i just cant figure it out
thanks
and also if you can explain to me the whole deal with center of mass in relativistic momentum and energy
when does the conservation of energy apply?
what happens in the center of the mass as oppose of out of it?

2. Jan 29, 2008

Dick

Energy and momentum are always both conserved. So the total momentum after the decay is zero. So the momentum of Q is equal in magnitude to the momentum of the massless particle. Now apply conservation of energy. And apply m^2*c^4=E^2-P^2*c^2 to each particle. You should basically get two equations in two unknowns.

3. Jan 29, 2008

Shooting Star

Suppose p1 is the momentum of the massless particle to the left and p2 is the mom of the particle with rest-mass Q to the right, with speed v. Let g denote gamma(v).

p1=p2=Qgv => Also, from energy consvn,

p1c + Qgc^2 = Mc^2 => (cancel c)
Qgv + Qgc = Mc. From this, the value of v comes out in terms of M, Q and c. Not difficult if you factorize and use compodendo-dividendo. Do the algebra. We get,

v/c = (M^2-Q^2)/(M^2+Q^2). Then,

p2 = Q*g*v, all expressed in terms of M, Q and c.