1. The problem statement, all variables and given/known data A lever of length 3L can rotate freely around a horizontal axis O. The end of each arm "2L" and "L" of the lever has one mass : "4m" and "9m" , respectively. Find the velocity of mass "9m" at the bottom. Consider that the lever was initially at rest in the horizontal position. 2. Relevant equations The problem is in a chapter of work and energy, before the conceps of torque and moment of inercia, so i guess that only energy and work can solve this. But any solution/hint with any method will be welcome. 3. The attempt at a solution I'm trying to apply conservation of energy, but i'm confused if the whole system is conservating energy or only part of it. I really need a hint of how start this problem or if possible a solution of it. Thanks in advance!