# Finding second moment of inertia about x axis

#### werson tan

1. 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

2. Homework Equations

3. The Attempt at a Solution
i' m using the formula Ixx = Ix +A(d^2)

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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#### werson tan

Can anyone tried to ans? I need it urgently.

#### SteamKing

Staff Emeritus
Homework Helper
1. 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

2. Homework Equations

3. The Attempt at a Solution
i' m using the formula Ixx = Ix +A(d^2)

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?

#### werson tan

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??

#### SteamKing

Staff Emeritus
Homework Helper
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) ?

"Finding second moment of inertia about x axis"

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