# Applied optimization to SHM problem? Hard?

1. Sep 10, 2014

### PhysicsKid0123

[Note: Post restored from archive - gneill, PF Mentor]

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

A pendulum consists of a uniform circular disk of radius r which is free to turn about a horizontal axis perpendicular to it's plane. Find the position of the axis for which the periodic time ( for small amplitude oscillations) is a minimum.

τ: torque
Ι (i): moment of inertia
m: mass
g:gravity
θ: angle between force mg and the radius length
ω: angular frequency
T: period
Sinθ=θ for small amplitudes.
L: length (offset for parallel axis theorem.) (unknown?)

2. Relevant equations

Equation (1): τ= -mgL θ

Equation (2): θ'' + (mgL/ I ) θ = 0

Equation (3): ω = (gL / (.5 r^2 + L^2)) ^.5

3. The attempt at a solution
So I solved for ω and my reasoning was that in order to have the minimum periodic time ω has to be at it's maximum. So I used applied optimization and set dω/dL = 0. However, I am not sure if this was the right way to do it since I squared both sides of the equation 3 to get rid of the square root and then used implicit differentiation. But I think this might be wrong because it's like if I differentiates the inside of Eq. 1 and that differentiation does not take into account the square root. So I am not sure if I did it right. I differentiated directly but the derivative was a monster and I couldn't factor out the L to solve for it. Is this a right way to do it correctly?

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Last edited by a moderator: Sep 10, 2014
2. Sep 10, 2014

### BvU

You removed the post ? I thought you did just fine!

3. Sep 10, 2014

### PhysicsKid0123

It was correct?

4. Sep 10, 2014

### PhysicsKid0123

Null

Last edited: Sep 10, 2014
5. Sep 10, 2014

### PhysicsKid0123

It was correct?

6. Sep 10, 2014

### BvU

Can't find no mistakes (PF isn't really meant for stamp approving, but I think you're doing fine).

Don't know of a different path to the same answer. Your squaring is justified: $\omega$ is known to be > 0 , so no problem.

7. Sep 10, 2014

### PhysicsKid0123

Oh alright. Got it. Thanks for your guidance then ;p