Anonymous

# Angular velocity of a yo-yo!?

1. The problem statement, all variables and given/known data

A certain yo-yo can be modeled as a uniform cylindrical

disk with mass M and radius R and a lightweight hub of

radius ½R. A light string is wrapped around the hub.

(a) First, the yo-yo is allowed to fall. Find the angular

velocity of the yo-yo when the string has unwrapped a

distance L.

(b) Now, imagine that that you pull upward on the string such

that the yo-yo remains in the same place. Find the angular

velocity of the yo-yo when you have pulled the string

upward a distance of L.

(c) Explain in words why it makes sense that the answers to

parts (a) and (b) are different.

2. Relevant equations

K (total) = .5 * I (center of mass) *w^2 + .5MR^2

I cm for a uniform cylindrical hub = .5M(R^2 + (.5R)^2)

3. The attempt at a solution

(A)

K (total) = .5 * I (center of mass) *w^2 + .5MR^2 = MgL

W^2 = MgL/ (.5 *I (cm) + .5MR^2)

I cm for a uniform cylindrical hub = .5M(R^2 + (.5R)^2)

So..

W^2 = MgL/(.5 * (.5M(R^2 + (.5R)^2) + .5MR^2)

W^2 = MgL/(1/4Mr^2 + 1/16MR^2 + 1/2MR^2)

W^2 = gL/(13/16R^2)

Does that seem about right? (obviously need to make it the square root but just leaving it squared for now). Actually maybe I should take into account the lightweight hub (but that is just a matter of adding another Inertia :)

(b)

I assume I cannot use conservation of energy, so maybe I could solve this with the Work that is done? I dunno, I'm confused I guess I don't know where to begin

Thanks for any help fellow physics buds!

Relevance