Homework Help: Rotation Problem

1. Dec 7, 2009

jamesdubya

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

A hoop, rolling at a constant velocity of 4m/s, rolls up a hill .8 m tall on to flat surface .
a) what percent of its initial total K.E. ( before rolling up the
hill) is due to rotational K.E.
b) how fast is it moving on top of hill.

2. Relevant equations
i=mr2
w=vr

3. The attempt at a solution
i am completely stumped, every equation i can find either requires a mass or a R or both. This problem looks like it should be easy thats why i feel so dumb for not knowing it.

2. Dec 7, 2009

Staff: Mentor

Just call the mass 'm' and the radius 'R'. You don't need actual values to answer the questions.

3. Dec 7, 2009

jamesdubya

well my study guide gave me the answers a)50 % b) 2.86. and i dont understand at all how they got that.

4. Dec 7, 2009

denverdoc

The plot deepens from that of the last problem--(man you guys are trying to bite off a lot at one time this w/e):

KE(total)=KE(translational)+KE(rotational)

KE(rotational)=1/2 I*w^2
KE(translational)=1/2 m V^2

now just pick an arbitrary R and m and put both equations in common terms; hint use w=vr and your other eqn I=mR^2. That should take care of probllem 1, but finish the other problem first. Mutitasking is good but not in physics!