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After a few hours of searching for anactivephysics message board, I was so happy to find this one and I am hoping for some assistance

I am not looking for homework answers, I have already completed this assignment to the best of my knowledge. I got my paper back today, and I am hoping to get help with some of the problems I got wrong. (textbook does not help memuchhere, these are teacher-created problems)

Okay, here we go

1) You have two bowling balls. One is 7 kg and the other is 10 kg. Which bowling ball has the greatest/least amout of rotational inertia and why?

My answer was that rotational inertia is found by (mass)(radius)squared. I reasoned that because both bowling balls have equal radius, the ball with the greater mass (10kg) would have a greater rotational inertia. My answer was marked completely wrong, however. :grumpy:

2) You have an upright, rotating pillar. Attached to the top of the pillar is a string and attached to the bottom of the string is a mass. As the pillar rotates faster, the mass becomes more and more horizontal.

1) Why does the mass travel in a circle?

2) Why does the mass gradually become more horizontal?

3) If the string were to break, which path would the mass take?

This is the one I had the most trouble with. I said that the mass travels in a circle because it has rotational inertia, and does not "want" to change its path. I said that it becomes more and more horizontal because the further from the axis the mass goes, the faster it has to move and the mass will speed up to match the speed of the spinning pillar. I said that if the string were to break, the mass would follow its original path (again because of rotational inertia). I am not sure if my answers were completely wrong or party right, or if the instructor wanted more.

3) You have a 10 m plank, a 100 kg boulder, and a 73 kg person. Ignoring the mass of the plank, where should you place the boulder so that the person may stand on the far end of the plank, with the plank extended 4 m over a cliff (so there are 6 feet on land)?

I used the formula (Fd)cw = (Fd)ccw here, and basicly made a mess of things :yuck: Guidance would be mucho appreciated here!

4) Again with the plank and boulder....Now the plank is 5 m long, and still extended 4 m over a cliff. What size mass do you need on the landside of the plank to keep the person (who is still at the far end over the cliff) from falling?

I said that the mass should be equal to the mass of the person. Wrong. I'm thinking now that I need to use (mass-system)(radius-centerofmass) = (mass1)(radius1) + (mass2)(radius2) or something...

Okay, thats it guys. ANY HELP OR INPUT is very appreciated, these problems are driving me nuts. Anyone that takes the time to work though this and reply is awesome, thank you so so much. Please, if you do reply, tell me which number you are referring to! Thanks!

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# Homework Help: Rotational Inertia assistance

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