# Homework Help: Can somone explain this review solution (vibrations)?

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1. Apr 30, 2017

### whitejac

1. The problem statement, all variables and given/known data

2. Relevant equations
The equations are all given

3. The attempt at a solution
This isn't really a hw question... it's solved, but i'm having a hard time following it. I don't understand where the first term:
-a(kaθ)

comes from. I can see it's the moment relation between the hinge and the spring but why is the a term squared?

2. May 1, 2017

### Nidum

Value 'a' is used to relate the force generated by the spring to the angle θ and as the lever arm length for the moment about the pivot generated by the spring force .

Last edited: May 1, 2017
3. May 1, 2017

### whitejac

Looking at it again. I'm sorry. I think my problem was just misreading the answer.
Value a just the value used to reference the length? (6in)

This would make:
F spring = xk
x = aθ

Thus a(kaΘ) = M
And it's negative because the fbd assumes the weight is tending CW while the weight and the spring force tend CCW?