1) A centrifuge rotor rotating at 10,300 rpm is shut off and is eventually brought uniformly to rest by a frictional torque of 1.20 m-N. If the mass of the rotor is 4.8 kg and it can be approximated as a solid cylinder of radius .0710 m, through how many revolutions will the rotor turn before coming to rest, and how long will it take?(adsbygoogle = window.adsbygoogle || []).push({}); correct answer: 993 rev, 10.9 s

So the first thing we are trying to find is rotational position (O) with a dash through it. And what we know is:

intial w = 10,300 rpm, bought to rest by 1.20 m-N torque, m = 4.8 kg, r = .0710 m & we know the moment of inertia equation of a solid cylinder which is 1/2MR^2

My thoughts we to find the moment of Inertia, then use the T = I(ang)a

so I = .0121, then use the torgue equation(1.20 mN/.0121) and ang. acc. = 99.2 m/s^2

then I could use rotational kinematics by: w2 = w(intial)^2 +2a(O)

10,300 rpm^2 = 0 + 2(99.2m/s)(O)

171.7 rps^2 = 198.4 m/s (O)

O = 149 revs... not the correct answer, so assuming I got that wrong I cannot get the time...

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2) A person stands, hands at his side, on a platform that is rotating at a rate of 1.30 rev/s. If he raises his arms to a horizontial position the speed of rotation decreases to 0.80 rev/s. (a) Why? (b)By what factor has the moment of inertia changed?(b) 1.6

Okay, so I know part a which is because his rotational inertia increases. Part b is the problem...

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# Homework Help: Problems w/ rotational dynamics and angular momentum

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