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A uniform disk of mass Mdisk = 5 kg and radius R = 0.2 m has a small block of mass mblock = 2.5 kg on its rim. It rotates about an axis a distance d = 0.17 m from its center intersecting the disk along the radius on which the block is situated.

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a) What is the moment of inertia of the block about the rotation axis?

b) What is the moment of inertia of the disk about the rotation axis?

c) When the system is rotating about the axis with an angular velocity of 4.7 rad/s, what is its energy?

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a) this part wasn't so bad. I=M*R^2 => (2.5kg)(.2m-.17m)^2=.0025kg*m^2

b) This is where I get stuck. The hint says to use the parallel-axis theorem, but I don't even understand it. Can someone explain it and show me how to use it for this problem? Greatly appreciate it.

c) Well, total energy is equal to the rotational kinetic energy of the block plus the rotational kinetic energy of the disk. So Total energy would be .5I*w^2 (disk)+.5*I*w^2 (block)

where w =omega.

I can figure out the KE for the blcok easily, but I don't know how to calculate the moment of inertia for the disk. Once I get that, I should have the answer down. :P

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# Homework Help: Disk and blockutter frustration

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