Moment of inertia of a thin, square plate

  • #1
simphys
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Homework Statement
the moment of inertia about an axis through the center of and perpendicular to a uniform, thin square plate. mass M and dimension L x L.
Relevant Equations
d
I don't really understand what the 2 integrals (dx and dxdy) for I_x represent. Could I get some explanation here please? Thanks in advance.
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  • #2
The second line in the derivation of ##I_x## has one ##dx## too many. Is that what bothers you?
 
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  • #3
Also, the second line is missing the ##r^2## and the limits of integration for ##x## are not correct. Keep in mind that this integral represents the moment of inertia about the x-axis, and the x-axis lies in the plane of the thin plate. Think about how to express ##r^2## in terms of ##x## and/or ##y##.
 
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  • #4
If you put in the correct $$r^2=x^2+ y^2$$ in the second line then you see that the x and y integrals give you the "perpendicular axis theorem" without issue. Please redo this.
 
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  • #5
TSny said:
Also, the second line is missing the ##r^2## and the limits of integration for ##x## are not correct. Keep in mind that this integral represents the moment of inertia about the x-axis, and the x-axis lies in the plane of the thin plate. Think about how to express ##r^2## in terms of ##x## and/or ##y##.
Yep, that is what I did I expressed in terms of y^2 and then went ahead with that. But I just didn't understand how this solution came about basically.
 
  • #6
hutchphd said:
If you put in the correct $$r^2=x^2+ y^2$$ in the second line then you see that the x and y integrals give you the "perpendicular axis theorem" without issue. Please redo this.
this is the solution. not mine :)
 
  • #7
So where is your attempt? How are we to help?
 
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  • #8
hutchphd said:
So where is your attempt? How are we to help?
well.. that is exactly what I don't understand..
I didn't understand why the solution has used two integrals in such a way.. I haven't used a volume integral or smtn. I did differently by using a mass element and summing over that with one definite integral.
 
  • #9
One must sum each mass element (mass density times volume element) over the entire volume of the object and scaled by the square of the distance to the chosen axis . The world is three dimensional and so is the integral. Show us your work.
 

FAQ: Moment of inertia of a thin, square plate

What is the moment of inertia of a thin, square plate about its central axis?

The moment of inertia of a thin, square plate of side length \( a \) and mass \( m \) about an axis perpendicular to the plate and passing through its center is given by \( I = \frac{1}{6}ma^2 \).

How do you derive the moment of inertia for a thin, square plate about its central axis?

The derivation involves integrating the mass distribution over the area of the plate. By dividing the plate into infinitesimal mass elements and summing their contributions, we find that \( I = \frac{1}{6}ma^2 \), where \( m \) is the mass and \( a \) is the side length of the plate.

What is the moment of inertia of a thin, square plate about an axis along one of its edges?

The moment of inertia of a thin, square plate about an axis along one of its edges is \( I = \frac{1}{3}ma^2 \), where \( m \) is the mass and \( a \) is the side length of the plate.

How does the orientation of the axis affect the moment of inertia of a thin, square plate?

The moment of inertia depends on the axis of rotation. For a central axis perpendicular to the plate, it is \( \frac{1}{6}ma^2 \). For an axis along one edge, it is \( \frac{1}{3}ma^2 \). The distribution of mass relative to the axis determines the moment of inertia.

Can the parallel axis theorem be applied to find the moment of inertia of a thin, square plate?

Yes, the parallel axis theorem can be used. If the moment of inertia about the center of mass is known (e.g., \( \frac{1}{6}ma^2 \) for a central axis), the moment of inertia about any parallel axis a distance \( d \) away can be found using \( I = I_{cm} + md^2 \).

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