GREPhysics.NET: GR0177 #25

GR0177 #25

Problem

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Mechanics $\Rightarrow$ }Moment of Inertia

The moment of inertia of the center penny about the center is just $1/2 m r^2$

The moment of inertia of any one of the other pennis about the center is given by the parallel axis theorem, $I=I_{CM} + md^2$ , where d is the distance from the new point from the center of mass. $I_{CM}=mr^2/2$ for each penny, and thus one has $I=mr^2/2+md^2=mr^2/2+m(2r)^2=9/2mr^2$ , since the distance from the center of each penny to the center of the configuration is 2r.

Since there are 6 pennies on the outside, one has the total inertia $I=1/2mr^2+54/2mr^2=55/2mr^2$ , as in choice (E).

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Comments

gmlaghari85
2014-08-13 00:49:34

Make it simple. Take any diagonal..

I = Icm + md^2= 1/2 mr^2 + Md^2 where M=3m on diagonal and d = 3r
then I = 1/2 mr^2 + 72mr^2 = 55/2 mr^2

gmlaghari85
2014-08-13 00:51:15

sorry its 27mr^2

Ac31415
2017-08-31 16:08:17

I understand this method. But why does it work??

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physik
2011-04-07 11:22:50

whats wrong with just doing this:rnrnIp:=I for the 6 penniesrnrnIp=6*(Mr^2)/2rnwe conclude that r=3R because thats how far each disk's boundery is from the center of the systemrn=> Ip= 6 $\frac{M}{2}$ *(3R)^2rn=> Ip= 6*(9) $\frac{M}{R^2}$ /2rnso Ip=(54/2)MR^2rnrnNow the center penny is just one disk Icp= (1/2)MR^2rnrnTotal I = Ip + Icp = (55/2)MR^2 Choice Ern

physik
2011-04-07 16:24:34

Im sorry about the look of this thing.
What I wanted to write was moment of inertia for the 6 disks is
Ip=6*(MR^2)/2

here R =3r => R^2 = 9r^2
Ip=((6*9)/2) Mr^2

and the moment of inertia of the central disk is Icp=MR^2/2

Total moment I = (MR^2)/2 + (54/2)(MR^2)

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anmuhich
2009-03-14 10:35:10

A faster way to do this is to treat the whole configuration as a disk and approximate the moment of intertia:

I=(1/2)(M)(R*R)

M= 7m and R= 3r

This gives I=(56/2)m *r*r which makes sense because this answer should be a little over the actual answer which is now obviously E

f4hy
2009-04-02 15:08:03

I wish I thought of that. Thanks.

mr_eggs
2009-08-16 21:23:26

but 9*7 is 63...

bcomnes
2011-09-22 16:11:53

It is an interesting way to get a ballpark number, but the number you actually get will not make sense if the answer choices include numbers between what this method gets and the actual correct answer.

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