I have noticed just in the past couple of years that I get a red splotchy rash on my neck when I drink beer. I thought it might be an allergic reaction, but it doesn't happen all of the time when I drink beer. I have tried switching to a different kind of beer, but I still have random outbreaks of this rash. Answer.
So, if two objects have the same energy $E_k$, they only have the same momentum if they also have the same mass. Since the bull has a much larger mass than the bullet, it must therefore have a much larger momentum than the bullet to arrive at the same kinetic energy. Force is change of momentum with time, $F = \dot p$.
In this case, the E in the formula is the energy released and removed, and the mass m is how much the mass decreases. In the same way, when any sort of energy is added to an isolated system, the increase in the mass is equal to the added energy divided by c2.