# Rotating disk falling from string problem

1. May 24, 2005

### sAXIn

Problem Sketch

Okay so this is the problem the upper disk will be of M1 mass and R1 radius
both disk are solid so I(oe)=1/2MR^2 , the second disk will be HM2 , HR2.
After system is left from rest I understand that the lower disc will start rotating and going down from the rope also it has linear acceleration down because the string is going down from upper disc.

I wrote sigma tau for both discs and sigma Y for the lower one but I need to express TA, Alfa 1 , Alfa 2 and A2 (linear acceleration of second disc)
(alfa -=> angular acceleration )! all of them should be written without being expressed by others.
I can't also figure out why when I express alfa2 by sigma moments on point where the string is leaving the disc it isn't equal to alfa2 by sigma moments on center of disc.

Thank's in advance for help .

#### Attached Files:

• ###### phys1.JPG
File size:
4.2 KB
Views:
165
2. May 24, 2005

### OlderDan

The upper disk only rotates, with angular acceleration resulting from the applied torque resulting from the tension in the string. The lower disk accelerates downward responding to the force of gravity less the tension in the string. It also rotates about its center because the tension results in a torque about the center. The thing the two disks have in common is the tension in the string. It sounds like you have written equations for these things. There is an additional connection between these three motions having to do with their displacements. Think about the distance the center of the lower disk has moved in terms of the angular displacements of the two disks.