1. The problem statement, all variables and given/known data Thanks for everyone that helped me with the physics questions! I had my physics 2 final today and got a 94!!! There was a question that I could not figure out for a long time. It goes like this. What would the velocity of the photon be if the Kinetic energy of the photon equals the rest energy of the photon? Express v in terms of c and anything else. Luckily, the question was a multiple choice, and the correct answer (which I guessed) was sqrt(3)/2 x c I still cannot figure out why this is true. 2. Relevant equations K=1/2mv^2 --> I assume this is the right equation to use... Er=mc^2 3. The attempt at a solution Equaling the two would give you: 1/2mv^2=mc^2 v^2=2c^2 v=sqrt(2) x c This is obviously not true since you can't go faster than the speed of light. Can someone explain to me how the answer is sqrt(3)/2 x c and why I am getting this thing wrong? Am I using the right Kinetic Energy equation?? Thanks!