1. The problem statement, all variables and given/known data Suppose that a particle of mass m and energy E is moving toward the origin of a system S such that its velocity u makes an angle alpha with the y-axis (approaches origin from upper right). Using the Lorentz transformations for energy and momentum, determine the energy E' of the particle measured by an observer in S', which moves relative to S such that the particle moves along the y' -axis. Please help, I can visualize how the problem is set up, but I don't know how to work it. Thank you. 2. Relevant equations I don't know if these are relevant, but: E^2=(mc^2)^2 + (pc)^2 p=gamma*mv E=gamma*mc^2 Px'=gamma(Px-(Beta*E/c)) Py'=Py Pz'=Pz E'/c=gamma(E/c-BetaPx) 3. The attempt at a solution I'm not really sure how to apply the equations to the problem. Please help.