hi.i've been asked to post this question here for a concrete answer..(adsbygoogle = window.adsbygoogle || []).push({});

the above question has been solved which the answer is 0.1706 reckk said: ↑1. The problem statement, all variables and given/known data

http://img232.imageshack.us/img232/4976/physicswe5.jpg [Broken]

total mass of the rod = 4m

m = 0.4kg; b = 0.4m; a = 1.0m, n = 800 U/min

rod is thin

i need to find the mass moment of inertia for the rotating rod relative to z-axis..

2. Relevant equations

(1/12)ML²

3. The attempt at a solution

for the rod which lies on y axis, i could calculate the mass moment of inertia by using the above equation which will lead me to the following answer

(1/12)*2*0.40*0.8² = 0.0427

but what should i do with the other parts of the rod which are parallel with the z-axis?..

or could i apply the following equation?:

(1/24)*M*L²*sin (2φ)

where φ = 45

thx in advance..

and here's the another question that i need a help

and here's my another approach reckk said: ↑here's the last question..

i need to find the bearing reaction on A and B

value given : n = 800/min; m = 0.4kg; a = 1.0m; b = 0.4m

here's my approach to solve the question:

Moment about x-axis:

Mx= 2 Jyz*ω²

= 2ω² ∫ b*(b/2) dm

= 2ω² ∫ b²/2 dm

= ω² * (b²/2) * m

ω = 2(pi)n/60 = 83.776 (1/s)

Mx= 83.776² * 0.4²/2 * 0.4

= 449.18 Nm

FA= FB=

Mx/2a

= 224.59 N

i wonder if i have done the right approach.. i took b/2 as its center of mass.. so i came up with following equation

∫ b*(b/2) dm

and since there's two parts which is parallel to z-axis.. i time the mass moment of inertia with 2..

is this the way to answer the question?.. i'm kind of confused with another method to calculate moment of inertia where the rod has an angle φ to the y-axis..

reckk said: ↑i came up with another approach

J = Jz1+ Jz2

= 2mr²

= 2*0.4*0.4²

= 0.128

Mx= Jω²

= 0.128 * (2(pi)n/60)²

= 898.35 N

FA = FB = Mx/2a

= 449.175 N

so..which one is the right approach? or is there any another approach?

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# Homework Help: Mass Moment of Inertia

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