Chronology Current Month Current Thread Current Date [Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

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

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.

https://youtu.be/Qc1JZ7l-_bM

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

--
*Please consider the environment before printing this e-mail*

__o

_ `\ <, _

...... ( • ) / ( • ) Lower Your Emissions, Raise Your Voice!
_______________________________________________
Forum for Physics Educators
Phys-l@mail.phys-l.org