A bullet of mass m and speed v hits a pendulum bob of mass M at time $t_1$  , and passes completely through the bob. The bullet emerges at time $t_2$ with a speed of v/2. The pendulum bob is suspended by a stiff rod of length l  and negligible mass. After the collision, the bob can barely swing through a complete vertical circle. At time $t_3$  , the bob reaches the highest position. What quantities are conserved in this process?