A uniform door of mass 22.5 kg, length 2.10 m, and width 0.900 m is kept in a vertical position by frictionless hinges on its length. A 135 g dove strikes the door at a point three-quarters of the width measured from the hinges with a horizontal speed of 20.0 m/s. The dove bounces off with a speed of 8.00 m/s in the opposite direction. State why angular momentum is a conserved quantity while linear momentum is not based on this collision.