# Where did ##\large \frac {4R}{3π}## come from?

Benjamin_harsh
Homework Statement:
Find the Moment of inertia of the shaded region w.r.t the line AB.
Relevant Equations:
##\large \frac {4R}{3π} \normalsize = 1.7 cm##

Sol: ##I_{AB} = I_{AB1} + I_{AB2} - I_{AB3}##

##I_{AB1} = I_{G1x} + A_{1}.Y_{2}##

moment of inertia of a triangle,##I_{XX} = \large \frac{bh^3}{36}##

moment of inertia of a triangle,##I_{YY} = \large\frac{hb^3}{36}##

##I_{AB1} = \large \frac {8*8^{3}}{36} + (\frac {1}{2})\normalsize*8*8 (\large\frac {8}{3})^{2} \normalsize = 341.33 cm^{4}##

##I_{AB2} = I_{G2x} + A_{1}.Y_{2}##

##\large \frac {4R}{3π} \normalsize = 1.7 cm##

Moment of inertia of semi circle section parallel to the base, ##I_{AB2} = 0.11*R^{4}##

##I_{AB2} = 0.11*44 + (\large \frac{π*4^4}{2})^{2}\normalsize (1.7)^{2}##

##I_{AB2} = 100.79 cm^{4}##

moment of inertia of a circle w.r.t centroidal X or centroidal Y: ##\frac {π}{64}d^{4}##

##I_{AB3} = I_{G2x} + A_{1}.Y^{2}##

##I_{AB3} = \large\frac{π}{64}\normalsize*4^{4}+(π*2^{2}).0##

##I_{AB3} = 12.56 cm^{4}##

##I_{AB} = I_{AB1} + I_{AB2} - I_{AB3} ##

##I_{AB} = 341.33 + 100.79 – 12.56 = 429.55 cm^{4}##

Last edited:

## Answers and Replies

Mentor
They are starting with the moment of inertia about the centroid of the semicular section, and then applying the parallel axis theorem. The centroid is located ##\frac{4R}{3\pi}## below the line AB.

Benjamin_harsh
So centroid of any semicircular section is always located at ##\frac{4R}{3\pi}##?

Mentor
Benjamin_harsh
Benjamin_harsh
In this equation, ##I_{AB1} = \large \frac {8*8^{3}}{36} + (\frac {1}{2})\normalsize*8*8 (\large\frac {8}{3})^{2} \normalsize = 341.33 cm^{4}## did they use both ##I_{XX}## and ##I_{YY}## values in it?

Mentor
did they use both ##I_{XX}## and ##I_{YY}## values in it?
No. Why would they do that?

Benjamin_harsh
No. Why would they do that?
So what values have they substitute in this formulae: ##I_{G1x}##?

Mentor
In all cases, they are applying the parallel axis theorem. They take the moment of inertia about the center of mass and parallel to the axis of rotation (the line AB) then add ##mD^2##.

Benjamin_harsh
In this equation for full circle, ##I_{AB3} = \large\frac{π}{64}\normalsize*4^{4}+(π*2^{2}).0##

Why they took zero as distance between centroid and parallel axis ? I am failing at measuring the distance between the parallel axis and centroid. .Please explain clearly.

Mentor
Why they took zero as distance between centroid and parallel axis ?
The center of the circle is on the axis.

Benjamin_harsh
The center of the circle is on the axis.
how can you tell?

Mentor
how can you tell?
Just by looking at the diagram.

Benjamin_harsh
Just by looking at the diagram.

In circles, axis always lies on the centroid?

Benjamin_harsh
Why they didn't calculate ##I_{XX}## and ##I_{YY}## for each 3 sections; right angled triangle, semicircle & full circle? In this problem, ##I_{XX}## and ##I_{YY}## calculated for each given 3 sections.

When should we calculate ##I_{XX}## and ##I_{YY}## for each sections?

Homework Helper
Gold Member
2022 Award
Why they didn't calculate ##I_{XX}## and ##I_{YY}## for each 3 sections; right angled triangle, semicircle & full circle? In this problem, ##I_{XX}## and ##I_{YY}## calculated for each given 3 sections.

When should we calculate ##I_{XX}## and ##I_{YY}## for each sections?
You have a lamina in the XY plane.
IXX is the MoI about a line through the centroid and parallel to the X axis. Likewise, IYY is about a line through the centroid and parallel to the Y axis.
For a lamina, you can find IZZ simply by adding these together. This is the perpendicular axis theorem.
In the question in this thread you are only asked for IXX, so you do not need to find IYY.

Benjamin_harsh
In the question in this thread you are only asked for IXX, so you do not need to find IYY?

How can you tell that no need to find IYY?

Mentor
How can you tell that no need to find IYY?
You are asked to find the moment of inertia about the line AB, which is parallel to the x-axis. So all moments of inertia needed for this problem will be parallel to that axis.

Benjamin_harsh
So all moments of inertia needed for this problem will be parallel to that axis.

So we need to neglect ##I_{YY}## and consider ##I_{XX}##?

Homework Helper
Gold Member
2022 Award
So we need to neglect ##I_{YY}## and consider ##I_{XX}##?
It's not a matter of neglecting IYY. You are only asked to find the moment about AB. That is a line parallel to the X axis, so you only need to find IXX and apply the parallel axis theorem; you do not need to know IYY.

Benjamin_harsh
Benjamin_harsh
So When should we to need calculate both ##I_{XX}## and ##I_{YY}##?

Homework Helper
Gold Member
2022 Award
So When should we to need calculate both ##I_{XX}## and ##I_{YY}##?
I already explained that in post #15 above. If you have a lamina in the XY plane and you need to find the MoI about an axis in the Z direction then one way to do that is to find IXX and IYY and add them together to get IZZ.

If the axis in the Z direction is not through the centroid then you can use the parallel axis theorem either separately for the XX and YY directions before adding or apply it after finding IZZ; the result is the same.
E.g. if the centroid is at (x,y) but you want the MoI about the Z axis (IZ0) then you can do IX0=IXX+My2, IY0=IYY+Mx2, IZ0=IX0+IY0
or
IZZ=IXX+IYY, IZ0=IZZ+M(x2+y2)

In post #14 above you linked to a problem which, according to your post #1 in that thread, asked for the MoI about "the centroidal axis". As I mentioned in post #12 in that thread that is not well defined. A centroidal axis is any axis through the centroid, not necessarily normal to the plane. If that question does intend IZZ then finding IXX and IYY is a good way. But as I noted, the diagram there does not provide any X coordinates and does not look symmetric, so I still think it may only have been seeking IXX.

In this thread it definitely only asks for IXX, so you do not need IYY.

Benjamin_harsh
If you have a lamina in the XY plane and you need to find the MoI about an axis in the Z direction then one way to do that is to find IXX and IYY and add them together to get IZZ.

How can I tell lamina is in the XY plane?

Homework Helper
Gold Member
2022 Award
How can I tell lamina is in the XY plane?
I am defining the XY plane as the plane containing the lamina.

Benjamin_harsh
What does ##X## and ##Y## indicates in this equation: ##I_{Z0} = I_{zz} + M(x^{2} + y^{2})##? Are there distance between centroid and the axis?

What value should I put in ##M##?