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Hi, i'm investigating a compound pendulum and was wondering if this isn't a stupid idea :tongue2:

I have a meter ruler with 2 flat cylinder 1 kilogramme weights sandwiched at the bottom of it securely fastened with a load of cellotape . The ruler has holes every 5 cm or so, and i found the centre of mass of the ruler is about 21cm.

I was planning to use the [tex]fn = 2\pi^-^1\sqrt(mgrIo^-^1)[/tex] and came to realise that i hadn't a clue what Io was, looked it up, and had a go at trying to find something to work it out. I thought if i found the moment of inertia for both the ruler and the weights separately and added them together it'd be ok

So i found quite a few formulas for rigid bodies, one of them looked like a general one but i didn't know how to do the integration, so i found this one for a ruler shaped object and thought hurray!

[tex]Icm = \frac{1}{12}mr(a^2 + b^2)[/tex]

Then realised it was only for an axis running through the centre of mass, but i was going to have the axis at different spots, but then found this other formula which seemed to sort that out and thought hurray! Since all the holes drilled are along the middle then if i used a hole not running through the centre of mass, it'd be parallel to the axis that could run through the centre of mass right?

[tex]I = md^2 + Icm[/tex]

And then i was dead tired, and thought it'd just be ok if i used the general

[tex]I = mr^2[/tex]

For the weights, and so if i added both of those I values together would that give me a okish value for Io or is it all terribly wrong :uhh: ?

Cheers

Richy

I have a meter ruler with 2 flat cylinder 1 kilogramme weights sandwiched at the bottom of it securely fastened with a load of cellotape . The ruler has holes every 5 cm or so, and i found the centre of mass of the ruler is about 21cm.

I was planning to use the [tex]fn = 2\pi^-^1\sqrt(mgrIo^-^1)[/tex] and came to realise that i hadn't a clue what Io was, looked it up, and had a go at trying to find something to work it out. I thought if i found the moment of inertia for both the ruler and the weights separately and added them together it'd be ok

So i found quite a few formulas for rigid bodies, one of them looked like a general one but i didn't know how to do the integration, so i found this one for a ruler shaped object and thought hurray!

[tex]Icm = \frac{1}{12}mr(a^2 + b^2)[/tex]

Then realised it was only for an axis running through the centre of mass, but i was going to have the axis at different spots, but then found this other formula which seemed to sort that out and thought hurray! Since all the holes drilled are along the middle then if i used a hole not running through the centre of mass, it'd be parallel to the axis that could run through the centre of mass right?

[tex]I = md^2 + Icm[/tex]

And then i was dead tired, and thought it'd just be ok if i used the general

[tex]I = mr^2[/tex]

For the weights, and so if i added both of those I values together would that give me a okish value for Io or is it all terribly wrong :uhh: ?

Cheers

Richy

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