Article on Pediatric MS- Momentum magazine

[Seasha]

Hello to those of you who have young ones w/ MS. I just read an interesting article in Momentum magazine called "When a Kid has MS" Don't know if any of you get this mag from NMSS, but you can read it online at nationalmssociety.org/magazine (pages 20-30)


It gives out lots of information and resources to get connected to!! Just thought maybe someone might be interested and hope it might help you and your kids.

[laurens]

Hello everyone,

One of the questions in passage VIII (about angular momentum/moment of inertia) is driving me crazy! Here is the set-up: 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.

The question (#53 in the 2008 edition) states that with his arms outstretched, the student drops the weights. This will cause the angular velocity of the student to:

A. increase
B. decrease
C. remain the same
D. A change cannot be determined.

I chose increase (choice A), but the answer says that it is should be choice C (remain the same). The explanation states that dropping the weights does not change the moment of inertia for the system.

I don't understand the answer/explanation at all -- I thought dropping the weights would decrease the mass of the system by 10kg. (I'm assuming that the weights hit the floor since the student had his arms outstretched when he dropped them and is sitting on a rotating stool.) The moment of inertia is proportional to the mass times the radius squared. So the moment of inertia should decrease and therefore the angular velocity should increase, right?

Maybe I'm missing something or reading the question wrong somehow? The only explanation I can think of is that the weights didn't hit the floor and somehow remained part of the rotating system at the same radius from the axis of rotation. But that seems like a huge stretch based on the wording of the question.

Any help would be much appreciated!!

Thanks in advance!!