A 99.9-kg horizontal circular platform rotates freely with no friction about its

Question

A 99.9-kg horizontal circular platform rotates freely with no friction about its center at an initial angular velocity of 1.53 rad/s. A monkey drops a 9.27-kg bunch of bananas vertically onto the platform. They hit the platform at 4/5 of its radius from the center, adhere to it there, and continue to rotate with it. Then the monkey, with a mass of 20.9 kg, drops vertically to the edge of the platform, grasps it, and continues to rotate with the platform. Find the angular velocity of the platform with its load. Model the platform as a disk of radius 1.57 m.

Explanation / Answer

Let’s use the following equation to determine the moment of inertia of the disk.

I = ½ * m * r^2 = ½ * 99.9 * 1.57^2 = 123.121
Initial angular momentum = 123.121 * 1.53 = 188.376

For the bananas, r = 4/5 * 1.57 = 1.256 m
I = 9.27 * 1.256^2 = 14.623
For the monkey, I = 20.9 * 1.57^2 = 51.516

Total moment of inertia = 123.121+ 14.623 + 51.516 = 189.26
Final angular momentum = 189.26 *

189.26 * = 188.37
= 188.37 ÷ 189.26

w = 0.995 rad/s

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