1. The problem statement, all variables and given/known data A particle flies along in the positive +x direction. It has a constant force F applied 30º clockwise to the x-axis. It is moving at .6 c. What is the angle of acceleration? 2. Relevant equations a = F/(mγ3) 3. The attempt at a solution I'm pretty sure I know how to do this problem. I expect that arctan-1(ay/ax) should give me the answer. Now for ax, I'm pretty sure the equation above works fine. It's what my textbook gave me. I also agree with the derivation. Nonetheless, it only applies to particles accelerated parallel to the direction of the velocity. So ax = Fsin(30)/mγ3) Now, I'm almost certain that the equation should be different for ay. Otherwise, ay/ax would just have a lot of cancellation and be sin(30)/cos(30) = tan(30) --> angle of acceleration = tan-1(tan(30)) = 30, which I don't think is right (otherwise the question would be a useless one to ask). I tried a couple of different things to find ay, but I couldn't find anything anywhere (be it online or in my head) that told me what the effect of velocity is on a acceleration when it is parallel to the force.