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kk727
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Homework Statement
This is a question from the 1999 AP Physics C test.
You can view the picture here, on Page 6:
http://vhphysics.com/ap/x-test-q/Resource/b)%20Physics%20C%20Materials%20(656)/c)%20AP%20C%20Free-Response%20Problems%20(213)/Physics%20C%201999%20Free%20Response.PDF
As shown above, a uniform disk is mounted to an axle and is free to rotate without friction. A thin uniform rod is rigidly attached to the disk, so that it will rotate with the disk. A block is attached to the end of the rod. Properties of the disk, rod, and block are as follows:
Disk: mass=3m, radius=R, moment of inertia about center ID=1.5mR2
Rod: mass=m, length=2R, moment of inertia about one end IR=4/3(mR2)
Block: mass=2m
The system is held in equilibrium with the rod at an angle θ to the vertical, as shown above, by a horizontal string of negligible mass with one end attached to the disk and the other to a wall. Express your answers in terms of m, R, θ, and g.
a. Determine the tension in the string.
The string is now cut, and the disk-rod-block system is free to rotate.
b. Determine the following for the instant immediately after the string is cut.
i. The magnitude of the angular acceleration of the disk.
ii. The magnitude of the linear acceleration of the mass at the end of the rod
As the disk rotates, the rod passes the horizontal position shown above.
c. Determine the linear speed of the mass at the end of the rod for the instant the rod is in the horizontal position.
Homework Equations
Not even sure.
The Attempt at a Solution
Again, the pictures are on that website.
I looked at this problem and my brain just wanted to explode. While we learned about inertia, tension, etc. in class, we learned about each individually, and not this in-depth. Trying to put everything together and understand it both conceptually and mathematically has been extremely difficult for me lately.
The solution to this question begins here, on Page 7:
http://vhphysics.com/ap/x-test-q/Resource/b)%20Physics%20C%20Materials%20(656)/d)%20AP%20C%20Rubrics%20(329)/Physics%20C%201999%20Scoring%20Guide.PDF
I looked it over, trying to follow the math, but I still do not understand what the heck is going on here. I get that the forces in the rod and block are going to equal the tension in the string - because it's not moving. But everything else is simply beyond me.
For example, that website says the tension in the rod is mgRsinθ. Why wouldn't it be 2R, since that's what the given length of the rod is?
I'm just extremely confused, and any explanations would be sooooo appreciated! Thanks in advance!
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