Ball a of mass m is connected to one end of a massless stick
with length `, the other end of which is connected to a pivot.
the stick is held vertically above the pivot and is then given
an infinitesimal kick. it swings down, and at the bottom of its
motion it collides elastically with ball b, also of mass m, which
hangs from a string with length ` and which is initially at rest.
ball b picks up whatever speed ball a had and then swings
upward in circular motion.
(a) a what point does ball b leave its circular motion and enter
freefall projectile motion?
(b) how high does ball b go in the resulting projectile motion