A disk rotates freely on a vertical axis with an angular velocity of 50 rpm. An identical disk rotates above it in the same direction about the same axis, but without touching the lower disk, at 20 rpm. The upper disk then drops onto the lower disk. After a short time, because of friction, they rotate together. The final angular velocity of the disks is1. 49 rpm2. 21 rpm3. 77 rpm4. 35 rpm5. 42 rpm

Question

Physics

Kirsten Mcmahon

10 August, 10:33

A disk rotates freely on a vertical axis with an angular velocity of 50 rpm. An identical disk rotates above it in the same direction about the same axis, but without touching the lower disk, at 20 rpm. The upper disk then drops onto the lower disk. After a short time, because of friction, they rotate together. The final angular velocity of the disks is1. 49 rpm2. 21 rpm3. 77 rpm4. 35 rpm5. 42 rpm

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Perla · Answer 1

Final angular velocity is 35rpm

Explanation:

Angular velocity is given by the equation:

I1w1i + I2w2i = I1w1f - I2w2f

But the two disks are identical, so Ii = I2

wf can be calculated using

wf = w1i - w2i/2

Given: w1i = 50rpm w2i = 30rpm

wf = (50 + 20) / 2

wf = 70/2 = 35rpm