# Homework Help: Seesaw equilbrium

1. Apr 4, 2014

### fromage

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?

2. Apr 4, 2014

### BvU

First question: "put the other mass on the other side what would happen to the seesaw?"
Depends on how gently the other mass is put. With a thump would indeed transfer some momentum on the whole. Extremely gently would probably not overcome friction. You already stipulate that in your last paragraph.

Second question, first part: I would say yes: there is no preference for equilibrium in the real world, unless the centre of mass is below the pivot axis.
Second question, second part: (insofar as not adressed in the first question): the way you ask ("the ensuing motion") leads me to think you jump to conclusions (as if "no motion" is already excluded). Find a real seesaw and experiment with presence of mind!

3. Apr 4, 2014

### epenguin

Quite a good question because with the schematic diagram you often see, well I have, of a fulcrum, two horizontal arms with weights on them which you are told are in equilibrium because there are equal downward forces, would be equally in equilibrium with the arms tilted so you couldn't use it for weighing in the way you do - on the other hand if the weights were unequal then it would swing till the arms were vertical which a balance also doesn't do.