Calculating Moment of Inertia for a Rectangle

In summary, the second moment of area for a rectangle can be calculated by taking the top half of the rectangle, finding its area, and multiplying it by the distance from the centroid of the top half to the neutral axis, squared. This value is then doubled since there are two halves of the rectangle. However, the correct method for calculating the second moment of area involves integrating (y^2) b dy from -h/2 to h/2, where h is the height of the rectangle and b is the width. This accounts for the difference between multiplying and integrating, and the use of the root mean square for the radius of gyration. This clarification was provided by Willem.
  • #1
imstat
2
0
If I have a Rectangle h high, b wide, I thought I could calculate 2nd Moment of Area, I , about its neutral axis this way: taking the top half of the rectangle, the area is bh/2, and then multiplying this by the distance from the centroid of the top half to the neutral axis, squared ie (h/4)**2. Then because there are two halves of the rectangle, doubling the answer. This all gives I=(bh**3)/16.

My problem is reconciling this with the answer obtained by integrating
(y**2) b dy from -h/2 to h/2, which gives I=bh**3)/12.

I generally thought the concept of second moment of area was an area times the distance of the centroid of the area to an axis it is rotated about, squared. Now I'm not so sure.

Any assistance appreciated.
 
Physics news on Phys.org
  • #2
[tex] I=\int _{S} r^2 dS [/tex]

This is the definition, so your second calculation is correct.

I generally thought the concept of second moment of area was an area times the distance of the centroid of the area to an axis it is rotated about, squared

Although this looks much like the definition, and yields the correct units, it differs from it because you multiply instead of integrate. If you insist using multiplication of some sort of 'mean distance' (it's called the radius of gyration) with an area, you should not use h/4. Because the definition involves r^2 instead of r you should use the 'root mean square' instead of the mean distance for your radius of gyration. But calculating the root mean square involves another integration, so I'd just stick to the above integral...
 
  • #3
Thank you da Willem. My concepts were not translating to accurate definition. So thanks for clarifying this for me.
Imstat
 

FAQ: Calculating Moment of Inertia for a Rectangle

What is the formula for calculating moment of inertia for a rectangle?

The formula for calculating moment of inertia for a rectangle is I = 1/12 * m * (h^2 + b^2), where I is the moment of inertia, m is the mass of the rectangle, h is the height, and b is the base.

How is the moment of inertia affected by the dimensions of a rectangle?

The moment of inertia is directly proportional to the dimensions of a rectangle. This means that as the height and base increase, the moment of inertia also increases. Similarly, as the height and base decrease, the moment of inertia decreases.

Is the moment of inertia the same for all axes of rotation in a rectangle?

No, the moment of inertia is different for different axes of rotation in a rectangle. The moment of inertia is the highest for an axis passing through the center of mass and perpendicular to the plane of the rectangle.

How does the distribution of mass affect the moment of inertia for a rectangle?

The distribution of mass affects the moment of inertia for a rectangle by changing the location of the center of mass. The farther the mass is from the axis of rotation, the higher the moment of inertia will be.

Can the moment of inertia for a rectangle be negative?

No, the moment of inertia for a rectangle cannot be negative. It is a measure of an object's resistance to change in rotational motion and cannot have a negative value.

Similar threads

Replies
3
Views
2K
Replies
2
Views
1K
Replies
12
Views
1K
Replies
2
Views
2K
Replies
6
Views
5K
Replies
30
Views
6K
Replies
5
Views
3K
Back
Top