A carriage of mass M goes to the right at a speed v. There is a
stick vertically fixed on this carriage. A rope inextensible and
massless is suspended (initially vertically) from the top of the
stick, and a small mass µ is attached to the end of the rope. At t
= 0 the carriage M strikes another carriage of mass m and the two
carts continue to the right, glued together. There is no friction
and the collision is perfectly inelastic. (Contrary to what is
indicated in the figure, the rope is vertical and at the same speed
as M before the collision.) If we assume that M, m >> µ, what
would be the minimum speed of v so that the mass µ makes a complete
circular turn around the rope attachment point?