# Help with determining other mass of a relativistic collision

1. Oct 18, 2011

### dgresch

1. The problem statement, all variables and given/known data
A 3.000 u (1 u = 931.5 MeV/c2) object moving to the right through a laboratory at
0.8c collides with a 4.000 u object moving to the left through the laboratory at
0.6c. Afterward there are two objects, one of which is a 6.000 u object at rest.
Determine the mass of the other object

2. Relevant equations
CONSERVATION OF ENERGY
$\gamma$m1c2+$\gamma$m2c2=$\gamma$m3c2+$\gamma$m4c2

3. The attempt at a solution
Each gamma is different because of different speeds but for the purpose of time management and the fact that past this I don't know what to do, I left them as gamma.
$\gamma$2794.5Mev+$\gamma$3726MeV=5589Mev+$\gamma$m4c2

Am I supposed to assume both objects are at rest? Doesn't seem like that would be the case because the other object would just be 1u. Any thoughts anyone?

2. Oct 19, 2011

### fzero

You will also need to apply conservation of momentum. Together with conservation of energy, you will have two equations for the two unknowns $\gamma_4, m_4$.