Plank Sinatra
Onarax<Snipped quote by Crimmy>
I don't know all the variables but I think it describes what happens if something moves faster than the speed of light. Do I have that right?
According to Newtonian mechanics, the momentum of a particle is equal to its mass multiplied by its velocity. Momentum is also a conserved quantity, which is true in all reference frames related by Galilean velocity transformations. However, the Galilean transformations are inconsistent with relativity. Say you're moving at 0.9c with respect to Earth, and then you shoot out an object that moves at 0.95c with respect to you; you'd expect the velocity of the object to be 1.85c with respect to the Earth, which is impossible because c, the speed of light in a vacuum, is the speed limit in all reference frames. But because you can't just throw the conservation of momentum out the window, however, you have to account for why it still works at significant fractions of c. It's an alright approximation when velocity "u" is small, but when you get big it starts going wonky. Instead, you'll have to use the "true time" measured by the particle itself.
You'll have to use the Lorentz transformations (except using the velocity of the particle rather than the reference frame) to get relativistic momentum.