The force acting on a moving charge particle with mass m and charge q in a magnetic field B is the Lorentz force F= q(v x B), where v is it's velocity. Suppose thata particle moves in the (x,y) plane with a uniform B field in the z direction. Assuming Newton' second law, mdv/dt = F, show that the forceand velocity are perpendicular, and that both have constan magnitude. Hint: Find (d/dt)(v dot v). I don't know where to start. How do you take the derivative of v dot v?