Say a beam, 3m long from end to end, has a pivot placed at its centre of mass, at half its length i.e. 1.5m. Objects with weight 200N each are placed on both ends of the beam. The system is in equilibrium and thus no turning effect.
Now a force of 30N is applied on one of the ends until the beam turns 15deg to the horizontal. When this force is removed, the moments on both sides are balanced. But, the beam turns, until it becomes horizontal?! If resultant moment is zero, how can the object turn?
Is the moment really not zero, or is there some potential energy transferred to the beam which converts when the force is released?