# Moment of Inertia of rectangular plate

1. Dec 17, 2004

### shizzle

How do i find using integrals, I of a rectangular plate with sides a and b with respect to side b?

I know i have to use the equation I = integral of (a^2 dm) and i know
density = m/ab but i'm having trouble figuring out the mass element. Can someone tell me what the mass element is? and walk me through how to find I from that point?

2. Dec 17, 2004

### krab

It's not integral of a^2 dm; it is integral of r^2 dm, where r goes from 0 to a. dm is the mass of the increment between r and r+dr.

3. Dec 17, 2004

### shizzle

Thanks, you're right. Its r^2 dm. I'm still not sure what dm will be in this case though. I'm thinking...rho*a*db? or is it rho*a*da? I'm really confused.

4. Dec 17, 2004

### dextercioby

If u're calculating the moment of inertia wrt to side "b",then u should apply the definiton.By "rho" i assume u meant superficial mass density of the plate which is obviously [itex] \frac{m}{ab} [/tex].Find the mass of an element of surface of rectangular shape which has the width "dr" and the length "b".Then integrate the final result (after making all mutiplications) from 0 to "a".

Daniel.

EDIT:I maybe wrong,though.I haven't done such calculations in years.

Last edited: Dec 17, 2004
5. Dec 17, 2004

### shizzle

from following your suggestions, this is what i have so far:

I = integral (r^2) *dm
dm = rho*b*dr
therefore, I = r^2 * rho*b*dr

If you integrate this from 0 to a, you get
I = (m*a^2)/3b

Could someone please check this and let me know if this makes sense?

6. Dec 17, 2004

### dextercioby

I underlined the wrong part of your work.

Daniel.

7. Dec 17, 2004

### shizzle

The first underlined part was a typo and should read:
rho = m/ab. anyway, lets try this again:

I = integral (r^2) *dm
rho = m/ab
dm = rho*b*dr
therefore, dI = r^2 * rho*b*dr. plugging in rho we get
dI = (r^2 * m *b*dr )/ab

If you integrate this from 0 to a, you get
I = (m*a^2)/3

yay!! i think i got it. Someone please confirm:)

8. Dec 17, 2004

### dextercioby

Yap,it's okay.This time. It's quite curious it doesn not depend on both sides of the rectangle,right??But that's not weird,because you've chosed as rotation axis one of the sides of the rectangle.So that side (the length of it) does not enter the final result.If your rotation axis would have been different from any of the rectangle's sides,both sides would have made it through the final expression.U can convince of that by computing the MI for an axis which is perpendicular on the plane of the rectangle and passes thrugh any point of the rectangle.Both "a" and "b" will be in the final result.

Daniel.

9. Dec 17, 2004

### shizzle

So for example, lets say that i wish to calculate the MI for an axis perpendicular to plane of rectangle and passes through center of mass, this is how i'd go about it:

Using perpendicular axis theorem, Iz = Ix + Iy so
Iz = [m (a^2 + b^2)]/3

is this right?

What would happen then if the axis was still perpendicular to plane but passing through a corner?

10. Dec 17, 2004

### dextercioby

No,it's wrong.Try to apply the definiton of the moment of inertia.Imagine an infinitesimal rectangle of sides 'dx' and 'dy' at a square distance of 'x^{2}+y^{2}' from the center.The center of the rectangle is the meeting point of the axis system.

Daniel.

11. Dec 17, 2004

### shizzle

Isn't the point of the parallel axis theorem and perpendicular axis theorem to make such calculations simpler? Do you know how we could apply the 2 theorems to solve this problem?

12. Dec 17, 2004

### dextercioby

I think you mean Steiner's theorem.In this problem the two axis of rotation are not coplanar.So you cannot apply the theorem.Take my advice and do the calculations with the definition.

Daniel.

13. Dec 17, 2004

### shizzle

Okay, lets see...:)

I = r^2 dm --according to definition:)
rho = m/ab
dm = rho * da*db (since x and y are really a and b in this case--right?)
dI = r^2 * rho* da*db
dI = (r^2 * m * da* db )/ab

i will then have to do a double integration right? over limits 0 to a/2 and 0 to b/2 respectively?.. hmm..

14. Dec 17, 2004

### dextercioby

1.Put "x" and "y" back.It avoids confunsion.
2.The limits of integration are wrong.You're computing only for half of the rectangle.Think again.
3.Yes,double integration.

Daniel.

15. Dec 17, 2004

### dextercioby

Yes it is correct to pick those integration limits.
Yes,of course they get replaced,since it is a definite integral with limits.

Daniel.

16. Dec 17, 2004

### shizzle

assuming this is right, i have no clue how we'd find the MI when the axis is perpendicular to plane but passing through a corner instead of cm?

17. Dec 17, 2004

### dextercioby

It is right.Use Steiner's theorem,knowing in this case that the 2 axis of rotation are coplanar (are actually parallel to each other).The answer is easy to get.

Daniel.

18. Dec 17, 2004

### shizzle

I = Icm + mr^2
I = (M(a^2 + a^2))/12 + m (a^2 +b^2) ---I'm b=not sure what r to use.
I = the sum once i know what r is

19. Dec 17, 2004

### dextercioby

The distance between one corner of the rectangle and the center????

Daniel.

20. Dec 17, 2004

### shizzle

The distance between one corner of the rectangle and the center is going to be
(a^2 + b^2 )/4 right? so if i plug this into the equation as my 2, that shd work.

I'm using pythagorean theorem to find this r.