As shown in the diagram below, a proton (p), mass 1 u, travelling in the +x direction with
speed vpi, collides elastically, not head-on, with a stationary deuteron (d) with mass 2 u. (here u means
the atomic mass unit. 1 u = 1.66 x 10-27 kg, but you may not need this number if you can get it to cancel
out of your equations). after the collision, the proton is observed to be moving in the +y direction.