1. The problem statement, all variables and given/known data "Given F=dp/dt. If the force is parallel to velocity show that F=γ3ma." 2. Relevant equations F=dp/dt and p=γmv 3. The attempt at a solution Since the force is the first derivative of the momentum with respect to time, and γ and v both vary with time since there is a net force causing both quantities to change, take the derivative of p using multiplication rule: dp/dt = m[γ'v+γa] γ'=γ3*v/c2 so this would change dp/dt to dp/dt = m[γ3(v/c)2+γa] After this I can't think of anywhere to go that could simplify this to the desired equation F=γ3ma.