In special relativity this quantity's four-vector includes an "energy over the speed of light" term, and the time derivative 
In special relativity this quantity's four-vector includes an "energy over the speed of light" term, and the time derivative of its expectation appears in the common form of Ehrenfest's theorem. The fact that this quantity does not commute with position gives rise to Heisenberg's Uncertainty Principle. Newton's Second Law gives force as equal to the time derivative of this quantity, and it is conserved in both inelastic and elastic collisions. For 10 points, name this quantity whose change is equal to impulse and which is equal to mass times velocity.
In special relativity this quantity's four-vector includes an "energy over the speed of light" term, and the time derivative of its expectation appears in the common form of Ehrenfest's theorem.
linear {momentum}