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 P: 365 My English are pretty poor! I will try again to explain. From the conservation of momentum you can find the velocity $V_A$ of M, in terms of the velocity v of m. From the conservation of energy for the mass M, from A to B, you can find the velocity $V_B$ in terms of $V_A$, i.e. in terms of v. Now demanding $V_B=0$ in order the pendulum barely swings you can calulate v. Does it makes any sence?