New studies of the wave and Schrodinger equations by the speaker seem to reveal that the momentum balance law is equally important than the energy balance law, even more important than the energy balance law. The advantage of the energy balance law is obvious because it gives the energy which is coercive. However, we discover that there are also ways to obtain coercive estimates from the momentum balance law. Our new div-curl lemma is one way of achieving this goal. I will explain how to do this and its applications to nonlinear wave and Schrodinger equations, including the Schrodinger map flow in one space dimensions and radial minimal surface equation in \(1+3\) dimensional Minkowski space.