- #1
fluidistic
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Homework Statement
Hi PF,
See figure for a clear view of the situation.
(There's a cylinder attached to a ceiling so that it cannot move. There's a cord on it and the cord goes down until another cylinder is attached to it. So that the only cylinder that can move is the one at the bottom. I call it "second cylinder". There's friction between the cord and cylinders such that the cord cannot slide on the cylinders. The radius of both cylinder is R and their mass is worth M.)
1)What is the acceleration of the center of mass of the second cylinder?
2)How much is worth the tension in the rope?
3)What's the velocity of the second cylinder when it has moved a distance of 10R?
2. The attempt at a solution
I'm not even able to find part a).
For the second cylinder, [tex]\frac{d \vec P}{dt}=\vec F_e=M \vec a_{cm}=\vec P + \vec T_2 \Leftrightarrow \vec a_{cm} =\frac{\vec P + T_2}{M}=\vec g +\frac{\vec T_2}{M}[/tex].
Now I must find [tex]\vec T_2[/tex] in order to answer part a) so by doing it I'd solve part b).
We have that [tex]T_2=T_1[/tex].
For the first cylinder, [tex]\frac{d \vec P}{dt}=0\Leftrightarrow \vec F_e=0\Leftrightarrow \vec P + \vec T_1+ \vec N \Leftrightarrow \vec N =-\vec P - \vec T_1[/tex].
Now I chose a center of momentum in the center of mass of the first cylinder. I have that [tex]\vec L= \vec L_{\text{spin}}=I\vec \omega[/tex]. (I do that in order to find out the angular acceleration of the first cylinder and then the acceleration of the point where is exerted [tex]T_1[/tex]).
Hence [tex]\frac{d \vec L}{dt}=I\vec \alpha = Ri \wedge -T_1j=-Rt_1k \Leftrightarrow \vec \alpha =-\frac{RT_1}{I}k=-\frac{RT_1}{\frac{MR^2}{2}}k=-\frac{2T_1}{MR}k[/tex].
In a rigid body we have that the velocity of a point of the body is worth [tex]\vec v_p =\vec v_{cm}+\vec \omega \wedge \vec r \Rightarrow \vec a_p = \vec a_{cm}+ \vec \alpha \wedge \vec r+ \vec \omega \wedge \vec v = \vec \alpha \wedge \vec r = \vec \alpha \vec r =-\frac{2T_1}{MR}j \wedge Ri=-\frac{2T_1}{M}j[/tex].
Thus [tex]\vec T_1 =-2 T_1j[/tex] which is impossible.