1. The problem statement, all variables and given/known data A proton has an energy of 2.5 × 10−10 J. What force is needed to keep it going in a circle having a 1.0 km radius 3. The attempt at a solution so i tried to find v first. total energy E = 2.5 x 10-10 J =[tex]\gamma[/tex] mc2 so m = photon mass = 1.67 x 10-27 so i found [tex]\gamma[/tex] = 1.66 and v = 0.798c so when i sub into f = mv2/R , i get f = 9.57 x 10-14 but the answer is 1.6 x 10-13 which means that the photon speed is roughly 0.9++ c so anyone knows whats wrong? btw why can't i use E= hf, get f, convert to T =1/f, then use circumference / T = v ? i realise that v i get is faster than the speed of light. but which step is wrong in this attempt? thanks!