aspiringdoc09

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Aug 18, 2009
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This a momentum and torque question.

Experiment I: One student sits on a stool that rotates freely. He holds a 5-kg mass in each hand. Initially, the student has an angular velocity of 5 radians/sec with his arms in his lap.

45) In experiment 1, with his arms outstretched, the student drops the weights. How can it be explained that the student's angular velocity does not change?

A. The center of mass does not change for the student, so angular momentum is constant.
B. The position of the weights does not impact angular momentum.
C. The falling weights continue to move in their circular pathway as they fall, so they retain their same angular momentum.
D. The weights are rotating in opposite directions after they are dropped.

I had used POE to get it down to A and B, and I chose A. I never fathom that C was an option because I thought they were asking about the student's angular velocity not the weights.

Can someone explain why my choices are wrong and why C makes sense? I don't understand their explanation. Thank you.
 

osprey099

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Jan 27, 2011
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This a momentum and torque question.

Experiment I: One student sits on a stool that rotates freely. He holds a 5-kg mass in each hand. Initially, the student has an angular velocity of 5 radians/sec with his arms in his lap.

45) In experiment 1, with his arms outstretched, the student drops the weights. How can it be explained that the student's angular velocity does not change?

A. The center of mass does not change for the student, so angular momentum is constant.
B. The position of the weights does not impact angular momentum.
C. The falling weights continue to move in their circular pathway as they fall, so they retain their same angular momentum.
D. The weights are rotating in opposite directions after they are dropped.

I had used POE to get it down to A and B, and I chose A. I never fathom that C was an option because I thought they were asking about the student's angular velocity not the weights.

Can someone explain why my choices are wrong and why C makes sense? I don't understand their explanation. Thank you.
the total momentum of the kid + balls does not change. The passage probably says something like: neglect air resistance. If you neglect air resistance, then the balls will continue to swing around in a circular path with same angular momentum as they fall. Therefore, the Momentum(initial) = Momentum(final).
 
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aspiringdoc09

aspiringdoc09

7+ Year Member
Aug 18, 2009
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the total momentum of the kid + balls does not change. The passage probably says something like: neglect air resistance. If you neglect air resistance, then the balls will continue to swing around in a circular path with same angular momentum as they fall. Therefore, the Momentum(initial) = Momentum(final).
They gave an equation in the passage (torque = change in L/ change t). L is the angular momentum and is equal to rotational inertia and angular velocity (L =I*omega). The passage states is ther is no net external torque on a system then angular momentum is conserved. No mention on air resistance, but I became confused because the question stem asks about student and the answer reflects on the weights.

Why are A & B wrong? Also, if the weights fly off tangentially wouldn't the student's rotational inertia change, which would change the angular velocity to keep angular momentum constant?
 

qw098

zyzzbrah
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Oct 23, 2011
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Yes, I had answered A initially as well.

I believe their wording is very poor, and I wouldn't worry about it. The real MCAT won't have these "wording" issues.