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Mechanics}Conservation of Energy

Conservation of energy gives  Mg(H) = \frac{1}{2}Mv^2 + \frac{1}{2}I \omega^2. v=R\omega, and thus the equation becomes

 Mg(H) = \frac{1}{2}Mv^2 + \frac{1}{2}I (v/R)^2.

Thus, I=2(MgH-\frac{1}{2}Mv^2)R^2/v^2. (Note that the height of the center of mass is the same at both the end and the start, thus the extra bit of the potential energy MgR cancels out. Thanks to the user keflavich for this correction.)

Given v^2 = 8gH/7, one plugs it into the equation above to get

I=2(MgH  -M4gH/7)R^2/(8gH/7)=2(3MgH/7)R^2/(8gH/7)=3MR^2/4, as in choice (B).

See below for user comments and alternate solutions! See below for user comments and alternate solutions!
Alternate Solutions
Ning Bao
2008-02-01 08:11:02
Let R, m,h,g=1. Then you just have

1=1/2*8/7+1/2*I*8/7
1=4/7+4/7*I
I=3/4.
Alternate Solution - Unverified
Comments
Ning Bao
2008-02-01 08:11:02
Let R, m,h,g=1. Then you just have

1=1/2*8/7+1/2*I*8/7
1=4/7+4/7*I
I=3/4.
smokwzbroiplytowej
2008-10-21 11:27:20
Nice - sure helps save time! Caveat: you really can't set R=1 and h=1 simultaneously, since they are both measures of length.

Here it works rather nicely, because H cancels out as you could tell from the answers.
Alternate Solution - Unverified
neutrino
2007-10-31 03:28:11
You can make the calculation more easy by noting that the rotational inertia will look like

I = \alpha M R^2  ,

with \alpha some constant. Now, you just have to solve:

MgH =  \frac{1}{2} M v^2 (1 + \alpha)

This might save you some time. Because, 1.7 min..., man, that is not much time.
NEC
Richard
2007-10-30 18:38:31
A potentially useless note:

The solution given in (E) is found by NOT accounting for the translational kinetic energy.
NEC
Andresito
2006-03-05 00:00:42
Yosun, there is a typo on your solution. When you express I, the term MV^2 has a factor of 2 implicitly.

Recall the conservation of energy equation, multiply both terms by 2 and the "only" term that has now a factor of 2 is MgH.

The correct expression should have I=(2 MgH - MV^2) (r/v)^2

Yosun, thank you very much for working out the problems. If you are a girl I send you a big kiss :). I obtained the pdf from the yahoo-group. It also has the typo.
Typo Alert!
keflavich
2005-11-10 22:48:10
You don't need to consider MgR as part of the gravitational potential energy. When the cylinder reaches the bottom of the plane, its center of mass will remain at a distance R above the ground.
yosun
2005-11-10 22:54:19
keflavich: you're right that the center of mass stays at a height of R above the ground at both points. thanks for the correction.
Fixed Typos!

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