find the moment of inertia


by dreamz25
Tags: inertia, moment
dreamz25
dreamz25 is offline
#1
Mar8-12, 11:17 PM
P: 26
q) A disc of mass 'm' and radius 'R' is attached to a rectangular plate of the same mass, breadth R and elngth 2R as shown in the figure. Find the moment of inertia of this system about the axis XX' passing through the centre of the disc and along the plane.



please help me out...!!
Phys.Org News Partner Science news on Phys.org
Lemurs match scent of a friend to sound of her voice
Repeated self-healing now possible in composite materials
'Heartbleed' fix may slow Web performance
Simon Bridge
Simon Bridge is offline
#2
Mar9-12, 12:55 AM
Homework
Sci Advisor
HW Helper
Thanks ∞
PF Gold
Simon Bridge's Avatar
P: 10,916
Well it's the sum of the moments separately ... I think the parallel axis theorem is your friend here. It is a good idea to show some attempt at working it out if only to show us how you are thinking about the problem.
dreamz25
dreamz25 is offline
#3
Mar9-12, 06:35 AM
P: 26
what i did was found the moment of inertia of the rectangular plate about the axis perpendicular to it and passing through the com
i.e, I1 = M(2R^2 + R^2)/12 = 5{MR^2}/12
Again,
I2 = M.I of the circular disc about the axis passing through its centre and perpendicular to it + I1 + M(d)^2 where d is the distance between them

so using it the T.I = MR^2/2 + 5MR^2/12 + M(3R/2)^2
= 19/6 MR^2
So, M.I about the axis passing through the centre and in the plane of the disc = 19/12 MR^2

but its 31/12 MR^2 which the book says...
so please help...
and yea i m a beginner.. :P

tiny-tim
tiny-tim is offline
#4
Mar9-12, 06:56 AM
Sci Advisor
HW Helper
Thanks
tiny-tim's Avatar
P: 26,167

find the moment of inertia


hi dreamz25! welcome to pf!

oooh, wrong axis!
Quote Quote by dreamz25 View Post
Find the moment of inertia of this system about the axis XX' passing through the centre of the disc and along the plane.
the axis is in the plane of the diagram
Simon Bridge
Simon Bridge is offline
#5
Mar9-12, 07:19 AM
Homework
Sci Advisor
HW Helper
Thanks ∞
PF Gold
Simon Bridge's Avatar
P: 10,916
The one he want's is not, in so many words, in this list:
http://en.wikipedia.org/wiki/List_of_moments_of_inertia
... derive it from the result for a cuboid?
tiny-tim
tiny-tim is offline
#6
Mar9-12, 07:24 AM
Sci Advisor
HW Helper
Thanks
tiny-tim's Avatar
P: 26,167
no, from the perpendicular axis theorem for moment of inertia
dreamz25
dreamz25 is offline
#7
Mar9-12, 07:35 AM
P: 26
thankyou sir...
but can u please work it out.......!!
m not getting what u want to say actually (
Simon Bridge
Simon Bridge is offline
#8
Mar9-12, 07:43 AM
Homework
Sci Advisor
HW Helper
Thanks ∞
PF Gold
Simon Bridge's Avatar
P: 10,916
@tiny-tim: Doesn't it give the same result?
@dreamz25: we are not supposed to do your work for you - working out the result yourself is part of the homework.
Try looking up the terms used and following the links supplied.

It looks like you know the formulae but don't understand them.
You found the moment of inertia for the plate for an axis perpendicular to it, but you need the axis along it's length.
Then apply paralell axis theorem.

Add to the moment of inertia for a disk, same problem - you need an axis through the center but in the plane of the disk, not perpendicular to it.
Look at the diagram you have and compare with the ones accompanying the formulae you used.
tiny-tim
tiny-tim is offline
#9
Mar9-12, 07:48 AM
Sci Advisor
HW Helper
Thanks
tiny-tim's Avatar
P: 26,167
Quote Quote by dreamz25 View Post
m not getting what u want to say actually (
the formula you used is for the moment of inertia about an axis perpendicular to the plane of the disc or plate

but the question asks about an axis parallel to (in) the plane of the disc or plate
dreamz25
dreamz25 is offline
#10
Mar9-12, 07:56 AM
P: 26
That's what my problem is...?
Perpendicular axis theorem says
I = Ix + Iy
but if it would have been a square then it was easy to calculate M.I of the plate about the axis in the plane of it...
but i don't know how to do it for the rectangular plate...?
Please atleast give me some source or proper concept...!!
Afterall i showed whatever i could..!!
dreamz25
dreamz25 is offline
#11
Mar9-12, 07:57 AM
P: 26
Once we know the M.I of the rec. plate about the axis parallel to XX' then its easy to apply the parallel axis theorem and kill the question................. :{
dreamz25
dreamz25 is offline
#12
Mar9-12, 08:04 AM
P: 26
still undone.. :{
Simon Bridge
Simon Bridge is offline
#13
Mar9-12, 10:01 AM
Homework
Sci Advisor
HW Helper
Thanks ∞
PF Gold
Simon Bridge's Avatar
P: 10,916
If you are really stuck you can always just go back to the calculus.


Register to reply

Related Discussions
Find the Moment of Inertia Introductory Physics Homework 5
How do I find the Moment of Inertia? Introductory Physics Homework 1
How do you find the moment of inertia of a polygon? General Math 4
how do I find the moment of inertia for a curve? Introductory Physics Homework 8
find the moment of inertia Precalculus Mathematics Homework 5