So my question is when to apply conservation of angular momentum?When there is no external force ,right?But in the case below A mass m moves in a circle on a smooth horizontal plane with velocity v0 at a radius R0. The mass is attached to a string which passes through a smooth hole in the plane as shown. The tension in the string is increased gradually and finally m moves in a circle of radius R0/2 . The final value of the kinetic energy is A) mv20 B) 1/4 mv20 C) 2 mv20 D) 1/2 mv20 My teacher applied conservation of angular momentum.But I don't know why?As tension force is external force the angular momentum should not be conserved. Note:I am not posting this question in homework sectio because I know it's solution .I don't want method to solve rather I am having a conceptual doubt.