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I recall my teacher explaining this concept in class, but he did not go over how we would solve these types of problems:
Bob the monkey (20 kg) is on a small platform (2 kg), which is attached by massless rods (R = 3m) to a frictionless pivot, and initially at rest.
http://img59.imageshack.us/img59/5623/bobthemonkey9go.jpg
Bob would like to get the banana shown, which he could reach-- if only the platform were rotated a half-circle from its current location. Being a smart monkey, he starts spinning around his center of mass on the platform, and soon has his snack.
a. What is Bob's rotational inertia about his center of mass? (You make approximate him as a sphere of radius 0.5 m).
b. What is the rotational inertia of the system around the frictionless pivot? (Remember that Bob also moves with the platform. In this calculation, treat him as a point mass.)
c. If Bob spins himself with ana ngular speed of 70 rad/s, how fast do Bob+platform rotate about the frictionless pivot? How long does it take for Bob to get his snack?
For A, I used I = (20)(2.5)^2 and (22)(2.5)^2, and both answers were wrong. I don't know what else I can use to figure out the rotational inertia around Bob's center of mass.
For B, I tried I = (2)(3)^2, but this answer was wrong.
I haven't tried C, since I felt that I would again get a wrong answer, since the previous 2 parts are incorrect. Any help on this problem would be greatly appreciated! Thanks!
Bob the monkey (20 kg) is on a small platform (2 kg), which is attached by massless rods (R = 3m) to a frictionless pivot, and initially at rest.
http://img59.imageshack.us/img59/5623/bobthemonkey9go.jpg
Bob would like to get the banana shown, which he could reach-- if only the platform were rotated a half-circle from its current location. Being a smart monkey, he starts spinning around his center of mass on the platform, and soon has his snack.
a. What is Bob's rotational inertia about his center of mass? (You make approximate him as a sphere of radius 0.5 m).
b. What is the rotational inertia of the system around the frictionless pivot? (Remember that Bob also moves with the platform. In this calculation, treat him as a point mass.)
c. If Bob spins himself with ana ngular speed of 70 rad/s, how fast do Bob+platform rotate about the frictionless pivot? How long does it take for Bob to get his snack?
For A, I used I = (20)(2.5)^2 and (22)(2.5)^2, and both answers were wrong. I don't know what else I can use to figure out the rotational inertia around Bob's center of mass.
For B, I tried I = (2)(3)^2, but this answer was wrong.
I haven't tried C, since I felt that I would again get a wrong answer, since the previous 2 parts are incorrect. Any help on this problem would be greatly appreciated! Thanks!
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And a smart monkey!