# A very tought physics sum I am having trouble with.

Hi,

I am preparing for my exam and I am stuck on one question.The question is:

A hallow cylinder(hoop) is rolling on a horizontal surface with at a speed v=3.3m/s when it reaches a 15degrees incline . a) how far up the incline will it go?.
b) How long will it be on the incline before it arrives back at the bottom.

Thanks for anyone that helps.This is not a home work question, just a question i am trying to attempt from my text book.I know that answers though.

a)4.3m b)5.2s

Pengwuino
Gold Member
Hmm... I'm not really sure how a) can be correct. Using that incline and conservation of energy, it comes out to 2.15m according to my calculations. Did you possibly use mv^2 instead of 1/2mv^2?

Päällikkö
Homework Helper
Using energy conservation, I get 4.289 for a).

For b): Can you figure out the cylinder's acceleration?
Hint: The total acceleration of the cylinder is the same as the tangential acceleration.

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For part (a) energy conservation is a good plan. The total mechanical energy remains constant... so thinking to set the initial KE equal the final PE is conceptually correct. However, the KE of a rolling hoop is significantly different than the KE of a sliding block (or hoop). Review in your text the KE definition of a rolling hoop. note... If you have not yet explored rotational inertia in your class, you will have some trouble with this question.