seesaw equilbrium

this isnt really a homework question but this seems the most appropriate area since i feel the mechanics involved is fairly simple.

i was absent mindedly considering a seesaw with two equal masses at either end of a massless arm and the fulcrum in the middle of the arm.
in this scenario if you were to put on mass on one side first, resulting in one end dipping towards the ground and the other higher up, then put the other mass on the other side what would happen to the seesaw?
obviously in the real world the seesaw would come to an equilibrium where the arm was horizontal, but when i tried considering both moments and resolving forces i could not find the force that is required to start this system moving, and hence i concluded that the seesaw with one mass higher than the other was already in equilibrium.
have i done something wrong here or missed out a crucial detail, or is it just that in the real world due to inaccuracies and the fact that some momentum would be exerted onto the seesaw arm once the second mass was added, that the ensuing motion takes place?

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