
#1
Nov2906, 06:01 PM

P: 22

1. The problem statement, all variables and given/known data
solid, uniform disk of mass M and radius a may be rotated about any axis parallel to the disk axis, at variable distances from the center of the disk If you use this disk as a pendulum bob, what is T(d), the period of the pendulum, if the axis is a distance d from the center of mass of the disk? and The period of the pendulum has an extremum (a local maximum or a local minimum) for some value of d between zero and infinity. Is it a local maximum or a local minimum? 2. Relevant equations From the picture, I come up with the moment of inertia of the solid disk around its center of mass I = 1/2Ma^2 From the question, we are asked to find the period of the pendulum if the axis distance d from the center of mass. The period T for this is P= 2pi (sqrt L/g) where g is the gravitation force and L is the lenght. From my understanding is that because of the new lenght, we need to use the Parallel Theorem to find the new lenght I am not sure about this, so hope someone can help Iend = Icm + Md^2 Iend = 1/2Ma^2 + Md^2 So the period is P = 2pi (sqrt(( a^2 +d^2)/g)) But this is not correct. Thank 



#2
Nov2906, 06:16 PM

HW Helper
P: 3,225

Good thoughts, but your expression for the period doesn't seem correct. You may want to look at this: http://hyperphysics.phyastr.gsu.edu/hbase/pendp.html. Your period expression has to include the moment of inertia.




#3
Nov2906, 06:59 PM

P: 22

I = 1/2Ma^2 P = P= 2pi (sqrt 1/2Ma^2/(1/2Mgd^2)) == then we can cancel out the M to get P = 2pi (sqrt 1/2a^2/(1/2gD^2)) === cancel 1/2 === P = sqrt(a^2/gd^2) 



#4
Nov2906, 07:50 PM

Sci Advisor
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P: 3,033

Extreme Period for a Physical Pendulumhttp://hyperphysics.phyastr.gsu.edu...parax.html#pax 



#5
Nov3006, 08:53 AM

P: 22

Is this the correct moment of inertia of the disk about the pivot point. I= 1/2Ma^2 + Md^2 = M ( 1/2a^2 + d^2) So the period of the disk is P = 2pi (sqrt (M(1/2a^2 + d^2)) 



#6
Nov3006, 09:18 AM

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P: 3,033

http://hyperphysics.phyastr.gsu.edu/hbase/pendp.html 



#7
Nov3006, 11:17 AM

P: 22

P = 2pi(sqrt(a^2/2gd + d/g)) If you look at this, how can you determine whether it has a local maximum or local minimum for some value of d? 



#8
Nov3006, 11:35 AM

Sci Advisor
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P: 3,033




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