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# Re: [Phys-L] Ex: Rotational Motion Pulley Question

• From: John Mallinckrodt <ajm@cpp.edu>
• Date: Fri, 5 Mar 2021 16:14:00 +0000

Actually, I consciously I opted for “axle” because that’s what the statement of the problem (which I linked to) calls it (despite the problem title.) Moreover, it doesn’t really make sense to call it a “pulley” given the way it is being used.

On Mar 5, 2021, at 7:52 AM, Carl Mungan via Phys-l <phys-l@mail.phys-l.org> wrote:

﻿Probably John meant to say the *pulley* has a fixed moment of inertia. The string is massless. It changes radius only because it is thick string wound on top of itself.

On Mar 5, 2021, at 10:23 AM, John Mallinckrodt <ajm@cpp.edu> wrote:

The problem specifies (in physics speak) that the axle has a fixed moment of inertia.

John Mallinckrodt
Cal Poly Pomona

On Mar 5, 2021, at 7:05 AM, Michael Barr via Phys-l <phys-l@mail.phys-l.org> wrote:

﻿In Unit 7 of the AP Physics 1 student workbook. Question 7M : Massive Pulley
In the question there is a stationary pulley with a constant mass hanging
off. As the pulley unwinds the radius also decreases.
In part C of the question the answer key explains that rotational
acceleration will decrease as the pulley unwinds due to less Torque. I'm
confused here. Yes there is less torque but if Torque = I x α, and I =
1/2MR^2, then moment of inertiaA decreases even more, so doesn't angular
acceleration go up. A 1/2 drop in radius causes Torque to drop in half but
inertia to drop 4 times. So, doesn't alpha double (assuming we neglect mass
change).
Am I missing something?
Michael Barr

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