Feyman and layman explanation of energy conservation

[AntiElephant]

Skip to 29:50. Here Feynman is explaining how some laws are not independent of energy conservation. In this case he goes on to explain how instead of using the law of levers were can use energy conservation to see what weight an object needs to on one side be to balance (or be in a state where is tilts back and forth without problems)

However I'm unsure how he comes to explain it. He seems to conclude that the potential energy at balance should be the same as the potential energy when it's titled. Why is this exactly? I thought it should be the total energy we need to worry about? Actually in an analogy previously it seems quite clear that only the total energy should be conserved. I probably wouldn't have worried about it except for the fact it also plops out the right answer, W = 8lb, so I must be understanding something wrong.

When it's tilted there is maximum potential energy, when it returns to balancing point some of the potential energy is now rotational kinetic energy. Right? How can the potential energy always stay zero?

 

[pbuk]

His point is that when a lever is balanced it can very easily (i.e. by only adding enough energy to overcome friction) be rotated a small distance about the fulcrum. So as no energy has been added, the increase in potential energy of the mass that is raised at one end of the lever must equal the decrease of the PE in the mass that is lowered. It is assumed that the system is stationary before and after the rotation so that KE is zero.

 

[AntiElephant]

His point is that when a lever is balanced it can very easily (i.e. by only adding enough energy to overcome friction) be rotated a small distance about the fulcrum. So as no energy has been added, the increase in potential energy of the mass that is raised at one end of the lever must equal the decrease of the PE in the mass that is lowered. It is assumed that the system is stationary before and after the rotation so that KE is zero.

Okay. So when it oscillates to the horizontal position would we have to conclude that KE at this point is practically 0? Because if the PE is always 0 at all stages then it could not have gained any KE.

 

[pbuk]

Okay. So when it oscillates to the horizontal position would we have to conclude that KE at this point is practically 0? Because if the PE is always 0 at all stages then it could not have gained any KE.

No, it does not oscillate to the horizontal position, it remains in the displaced position because there is no change in PE. Perhaps you are thinking about a balance rather than a simple lever where there is a counterweight that gains PE when the balance arm is displaced.