# Homework about spinning top [closed]

I have a top of unknown mass that has a moment of inertia $I=4\times 10^{-7} kg \cdot m^2$. A string is wrapped around the top and pulls it so that its tension is kept at 5.57 N for a distance of .8 m.

Could somebody help me derive some equations to help with this? Or to get me in the right direction? I have been trying to derive some sort of equations from $E=\frac{I \cdot \omega ^{2}}{2}$ but I cant get anywhere without ending up at radius = radius or mass = mass.

I need the final angular velocity.

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So far you haven't asked a question. What are you trying to find? The kinetic energy of rotation? Torque = moment-of-inertia * angular-acceleration will give you the final angular velocity ('course you'll need to know the radius of the wrapping...). Can you find the energy from there? –  dmckee Feb 2 '11 at 23:02
Sorry I changed it to include what I need. Can I equate the pulling force to the rotational energy? 1/2 I W^2 –  Justin Meiners Feb 2 '11 at 23:06
Justin: No. The units are wrong. You can equate the work (units of energy, right?) done in spinning the top with the energy of rotation. –  dmckee Feb 2 '11 at 23:08
So could I use that Force * distance of the string and that would be energy? –  Justin Meiners Feb 2 '11 at 23:10
Try asking this at Yahoo questions where you will get some good answers. –  John McVirgo Feb 2 '11 at 23:17
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## closed as off-topic by tpg2114, Qmechanic♦Nov 3 '13 at 2:26

This question appears to be off-topic. The users who voted to close gave this specific reason:

• "Homework-like questions should ask about a specific physics concept and show some effort to work through the problem. We want our questions to be useful to the broader community, and to future users. See our meta site for more guidance on how to edit your question to make it better" – tpg2114, Qmechanic
If this question can be reworded to fit the rules in the help center, please edit the question.

You should be able to calculate the work done by pulling the string.

You should also be able to write down an equation for the amount of work necessary to accelerate an object with a given MOI to some arbitrary angular velocity.

That should be a good start.

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