Problem 1.162

A horizontally flying bullet of mass $m$ gets stuck in a body of mass $M$ suspended by two identical threads of length $l$ (Fig. 1.42 ). As a result, the threads swerve through an angle $\theta$. Assuming $m \ll M$, find: (a) the velocity of the bullet before striking the body; (b) the fraction of the bullet's initial kinetic energy that turned into heat.