Finding second moment of inertia about x axis

AI Thread Summary
To find the second moment of inertia about the x-axis for a cubic structure divided into four parts, the initial calculations using the formula Ixx = Ix + A(d^2) were incorrect. The discussion emphasizes that due to the symmetry of the structure, it may be simpler to calculate the moment of inertia by subtracting the moment of inertia of the interior from that of the exterior, rather than using the parallel axis theorem. The participants point out that the proposed calculations do not yield valid results, particularly questioning the logic behind using (25 - 25) in the formula. Overall, the correct approach involves reassessing the method to accurately determine the second moment of inertia. The conversation highlights the importance of verifying calculations in structural analysis.
werson tan
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Homework Statement


The cubic is divided into 4 parts , A, B and C, D , each with thickness of 1mm , i am sked to find thesecond moment of inertia about x -axis

Homework Equations

The Attempt at a Solution


i' m using the formula Ixx = Ix +A(d^2)[/B]
for part CD, Ixx = 50(1^3) / 12 + 50 ((0.5-0.5)^2)
or Ixx = 50(1^3) / 12 + 50 ((25-0.5)^2)
is correct ?
 

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Can anyone tried to ans? I need it urgently.
 
werson tan said:

Homework Statement


The cubic is divided into 4 parts , A, B and C, D , each with thickness of 1mm , i am sked to find thesecond moment of inertia about x -axis

Homework Equations

The Attempt at a Solution


i' m using the formula Ixx = Ix +A(d^2)[/B]
for part CD, Ixx = 50(1^3) / 12 + 50 ((0.5-0.5)^2)
or Ixx = 50(1^3) / 12 + 50 ((25-0.5)^2)
is correct ?
If you're trying to calculate the second moment of area about the x-axis, no, your calculations are not correct.

Hint: Since everything is symmetrical about the x- and y-axes and no material is located inside the 1 mm thick plates, wouldn't it be easier to dispense with the parallel axis theorem and just subtract the MOI of the interior from the MOI of the exterior?
 
SteamKing said:
If you're trying to calculate the second moment of area about the x-axis, no, your calculations are not correct.

Hint: Since everything is symmetrical about the x- and y-axes and no material is located inside the 1 mm thick plates, wouldn't it be easier to dispense with the parallel axis theorem and just subtract the MOI of the interior from the MOI of the exterior?
Ixx = 50(1^3) / 12 + 50 ((25-25)^2) this is the correct one??
 
werson tan said:
Ixx = 50(1^3) / 12 + 50 ((25-25)^2) this is the correct one??
Like your original calculations, that expression doesn't make sense

What is (25 - 25) ?
 

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