Dumbell problem (conservation of energy)

[terryds]

Homework Statement

A dumbell is leaned vertically on the wall. After it is deviated with some angle θ, dumbell starts sliding.
Determine the angle between the bar and the horizontal face (floor) when the upper mass starts to fall.

Homework Equations

E = E'

The Attempt at a Solution

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I don't understand how come there is mgL cos θ??
I think the potential should be mg L sin θ, right??

Please help

 

[kuruman]

Right. Do you know how to finish the problem?