You're going to have a problem with the result because in the example you gave, the answer is 1.7426e10 which is a 34 bit number. You can't cram that into an unsigned long. You will either have to store the result as a 64-bit integer or leave the result as floating point.
How are you going to use V? Your Velocity function is declared void and therefore can't have a return value and V is declared local to the Velocity function and can't be used outside it anyway.
Pete