Hello, Sometimes in introductory physics, I see this: After defining P as the momentum, we show: dP/dt = F Then, later, I see this ΔL = mv1 - mv2 So my question is really simple: WHY is there no consistency? Why do we sometimes use the Δ symbol? Is there something about how the definition is being applied (small change but not infinitesimally small), that makes the books switch back to Δ I see it here, too (where U is the potential energy in a non-dispersive system): ΔK.E. + ΔU = 0 Is there an APPLIED (as, say, in engineering) advantage to using the above, and not: K.E. + U = constant which, as I see it, is more theoretically precise.