Moments of Inertia of a Flat Body

In summary, the conversation discusses finding the moments of inertia of a flat body, specifically a rectangular body with uniform mass distribution. The question asks for the principal axes and moments of inertia, not just one moment of inertia. The conversation also mentions using a double integral for the calculation. It is noted that any axis of symmetry is a principal axis and that every body has a different moment of inertia about each axis. The question specifically asks for the moments of inertia about the principal axes.
  • #1
hamjam9
7
0
How do you take the moments of inertia of a flat body? I know howto take it if it's a 3d body. And the 2d case should be really simpe,but I'm too stupid to figure it out. Can you help me? For example.. Say we have a body that's a rectangle of mass m on |x| < a, |y| < b..? Thanks so much.
 
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  • #2
Hi hamjam9! :smile:

I assume you're given a mass-per-area instead of a density?

For a "flat" body (no such thing, really :rolleyes:), you just treat it as if it has a very very small thickness. :wink:
 
  • #3
Or perhaps the thickness doesn't matter. Could it be unit thickness? As long as the density doesn't vary with the z-coordinate of your example.

In any case, I usually find it easier to do a double integral [tex]\int \int r^2 \sigma(x,y) dx dy[/tex] than a triple integral [tex]\int \int \int r^2 \sigma(x,y,z) dx dy dz[/tex]
 
  • #4
hamjam9 said:
Hey tiny-tim. Thanks for answering my question about moments of inertia. I thought no one would answer it lol. I'm so sorry to PM you but I'm really desperate. I'll be honest, I have assignment due tmrw, and I can't get this one question done !:frown:

"Find the principal axes and moments of inertia of a flat homogeneous rectangular body (|x| ≤ a, |y| ≤ b) of mass m (uniformly distrubuted)."

The question asks to find the 'principal axes' and 'momentS of inertia'. Not just the 'moment' of inertia, but moments.

Now I know I should be taking my origin to be in the dead center of the rectangle, but I'm unsure about how to state what the principal axes are, and then how to get the 3 moments of inertias (I know 1 of those 3 is actually zero, since it's flat). If you could help me that would be so great. I'mso sorry to PM you man.

Hi hamjam9! :smile:

No, none of the moments of inertia is zero.

Look at the PF library on moment of inertia :wink:

Any axis of symmetry is a principal axis.

Any body has a different moment of inertia about every axis …

this question asks you for the ones about the principal axes. :smile:
 

1. What is the definition of moment of inertia for a flat body?

Moment of inertia for a flat body is the measure of an object's resistance to changes in its rotational motion. It is a physical property that depends on the mass distribution and shape of the object.

2. How is the moment of inertia calculated for a flat body?

The moment of inertia for a flat body can be calculated using the formula I = Σmr², where I is the moment of inertia, Σm is the sum of the mass of all the particles in the body, and r is the distance of each particle from the axis of rotation.

3. What are the factors that affect the moment of inertia of a flat body?

The moment of inertia of a flat body depends on the mass distribution and shape of the object. Objects with more mass located further from the axis of rotation have a larger moment of inertia. Similarly, objects with a larger surface area have a larger moment of inertia compared to objects with a smaller surface area.

4. How is the moment of inertia affected by the shape of a flat body?

The moment of inertia of a flat body is directly affected by its shape. Objects with a larger surface area or more mass located further from the axis of rotation have a larger moment of inertia. For example, a disc has a larger moment of inertia compared to a rod with the same mass and length.

5. Why is the moment of inertia important in physics?

The moment of inertia is an important concept in physics as it helps us understand the rotational motion of objects. It is used in many engineering and scientific applications, such as designing machines, calculating the torque required for an object to rotate, and predicting the behavior of rotating bodies. It is also essential in the study of angular momentum and the conservation of energy in rotational systems.

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