# Homework Help: Two Conceptual questions about rotational mechanics

1. Jul 14, 2012

### babayevdavid

Hi,

I see in many of these physics problems it is explicitly stated that "the cord on the pulley does not slip." [1] What is the implication of this? Why is it important?

Also, there is a problem here with a diagram that I will try to explain in words and then have a question about. (Assuming frictionless axes) There is a sphere rotating around a vertical axis through its center. Around its equator is tied some massless cord that continues horizontally from the sphere onto a pulley (I and radius of the sphere and pulley are given). The cord hangs over the pulley and at the end of the cord is a mass. If the mass were to be dropped a certain distance vertically from rest, and I wanted to find its final speed, and I used the sum of all kinetic energy present = the initial potential energy of the mass at rest, [2] is it okay to substitute the omegas in that setup for V/r ? Why is that okay? It is a valid assumption that the cord will have the same final speed throughout the system?