18.A particle of mass m is attached to a disc of equal mass m by means of a slack string as shown.The disc is hinged about its centre C on a horizontal smooth table.The particle  is thrown on the horizontal table with initial velocity v.Find its velocity , when the string becomes taut.


1)  V

2)$\frac{V}{3}$
3)$\frac{2V}{3}$
4)$\frac{3V}{4}$