A bob of mass m suspended by a light string of length L is whirled into a vertical circle as shown in figure. What will be the trajectory of the particle, if the string is cut at B.

 

When bob is whirled into a vertical circle, the required centripetal force is obtained from the tension in the string. When string is cut, tension in string becomes zero and centripetal force is not provided, hence bob start to move in a straight line path along the direction of its velocity. At point B, the velocity of B is vertically downward therefore, when string is cut at B, bob moves vertically downward.