PhysicsLAB AP Free Response Question
2001 C3

 
A light string that is attached to a large block of mass 4m passes over a pulley with negligible rotational inertia and is wrapped around a vertical pole of radius r, as shown in Experiment A above.
 
The system is released from rest, and as the block descends the string unwinds and the vertical pole with its attached apparatus rotates. The apparatus consists of a horizontal rod of length 2L, with a small block of mass m attached at each end. The rotational inertia of the pole and the rod are negligible.
 
 Determine the rotational inertia of the rod and block apparatus attached to the top of the pole.

 Determine the downward acceleration of the large block.

 
When the large block has descended a distance D, how does the instantaneous total kinetic energy of the three blocks compare with the value 4mgD?
 
___ Greater than 4mgD    ___ Equal to 4mgD    ___ Less than 4mgD
 
Check the appropriate space and justify your answer.

 
 
 
The system is now reset. The string is rewound around the pole to bring the large block back to its original location. The small blocks are detached from the rod and then suspended from each end of the rod, using strings of length .
 
The system is again released from rest so that as the large block descends and the apparatus rotates, the small blocks swing outward, as shown in Experiment B above. This time the downward acceleration of the block decreases with time after the system is released.
 
 
When the large block has descended a distance D, how does the instantaneous total kinetic energy of the three blocks compare to that in part c?
 
___ Greater before    ___ Equal to before    ___ Less than before
 
Check the appropriate space and justify your answer.





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