Calculating Kinetic Energy and Speed with Rolling Hoop Problem

[jamesdubya]

Homework Statement

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.

Homework Equations

i=mr2
w=vr

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 that's why i feel so dumb for not knowing it.

 

[Doc Al]

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

 

[jamesdubya]

well my study guide gave me the answers a)50 % b) 2.86. and i don't understand at all how they got that.

 

[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!