65. Although we usually write Newton's second law for one-dimensional motion in the form d mv F = ma, which holds when mass is constant, a more fundamental version is F = if! ). Consider an object whose mass is changing, and use the product rule for derivatives to dm show that Newton's law then takes the form F = mm + v3. 66. A railroad car is being pulled beneath a grain elevator that dumps grain at the rate of 450 kg/s. Use the result of Problem 65';I to nd the force needed to keep the car moving at a constant 2.0 m/ s