1. The problem statement, all variables and given/known data Hi, I have this problem: For motion under a pure (rest mass preserving) inverse square law force f = −αr/r3 , where α is a constant, derive the energy equation γmc2 − α/r = constant. 2. Relevant equations E = γmc2 dE/dt = f.u for a pure force 3. The attempt at a solution I start by saying: dE/dt = f.u = f.dr/dt Next, I know f is independent of time, so I can just integrate this to get: E = f.r +constant So then you get: γmc2 = -α/r + constant Which is painfully close to the result I'm meant to get but out by a minus sign. I don't know if I've made an error here or if the question is erroneous. Thanks for any help.