1. The problem statement, all variables and given/known data A thin uniform bar 2.00 m long and weighing 90.0 N is hanging from the ceiling by a frictionless pivot. It is suddenly struck 1.50 m below the pivot by a small 3.00 kg ball initially travellimg horizontally at 10.0 m/s. The ball rebounds and moves in oppossite direction with a velocity of 6 m/s. Find the angular speed of the bar after the collision. 2. Relevant equations I decided to use the principle of conservation of angular momentum. 3. The attempt at a solution And I got the answer pretty easily too. But, I did not understand why I could use it. I have read its valid when external torques sum up to 0. So can you explain how it is 0 in this case? And is the linear momentum conserved?