A bullet with mass mA and velocity vA makes a completely inelastic collision with a still pendulum of mass B. After the collision, the pendulum swings to a height of y from its equilibrium with the bullet in it. In terms of y, mB and mA, what is the initial velocity?
My textbook says that momentum is not conserved because of the external forces of tension and gravity acting on the system; this makes sense to me. But, in my book's work, it gets the answer by using Pinitial = Pfinal, where the initial is the bullet's mass multiplied by its velocity, and the final momentum is both masses multiplied by their velocity in the +x direction. How can they say that momentum is not conserved and then use the conservative equation?