# Moment of inertia questionsWatch

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#1
I would like some help with these two questions about moment of inertia please.

-First question: https://gyazo.com/eb3f9ef4cc667878c2534fa1624fd00c

-Second question: https://gyazo.com/acef516515e357b2e61f92d022228772

For the first question, my attempt was to divide the body into two sections ( one vertical rectangle and one horizontal rectangle). To moments from inertia from pivot 0 for vertical rectangle and then use the parallel axis theorem to find the moment of inertia for horizontal rectangle. (Picture below).

EDIT: Solved the first question, only need help for the second question.

For the second question, I'm sad to say I don't have an idea on how to even start the question...

Any help is appreciated.
Last edited by Jian17; 4 months ago
0
3 months ago
#2
(Original post by Jian17)
EDIT: Solved the first question, only need help for the second question.

For the second question, I'm sad to say I don't have an idea on how to even start the question...

Any help is appreciated.

You'll need to use the // axis theorem: axis yy offset by 0.294 m from being aligned with CoM of the elements (0.3m - 0.5*thickness). You can get moment of inertia of the main body (without any elements).

54 = I (body) + (2*15*0.35^2 + 2*15*0.294^2) + (2*15*0.9^2 + 2*15*0.294^2) = I (body) + 30[2*0.294^2 + 0.35^2 + 0.9^2). So I (body) = 54 - 30[...]

Then add I (new elements) to I (body). Same thickness and offset 0.294 m. Higher masses & lengths. To fit in the same body, shift CoM inwards; by 0.05m for outer elements, 0.15m for inner (don't think width matters).

Tagged ghostwalker, he probs knows best for this
ghostwalker
Last edited by Physics Enemy; 3 months ago
0
3 months ago
#3
(Original post by Jian17)
For the second question, I'm sad to say I don't have an idea on how to even start the question...
Physics Enemy For info.

Phsyics Enemy's method seems fine. Question gives you the methodology to use. They're ignoring the MofI of each component about its own CofM, presumably as it's a "quick" calculation.

I assume the components are central in their slots, so their length and width don't come into it. Thickness effects distance to yy-axis as per other post.

Not checking the arithmetic.
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