Disk A, with a mass of 9.0 kg and a radius of 30 cm, rotates clockwise about a f
ID: 3897115 • Letter: D
Question
Disk A, with a mass of 9.0 kg and a radius of 30 cm, rotates clockwise about a frictionless vertical axle at 20 rev/s. Disk B, also 9.0 kg but with a radius of 20 cm, rotates counterclockwise about that same axle, but at a greater height than disk A, at 20 rev/s. Disk B slides does the axle until it lands on top of disk A,
A) after which they rotate together. After the collison, what is their common angular speed (in rev/s)?
B) and in which direction do they rotate? (clockwise or counterclockwise)
Explanation / Answer
I1 = (9)(0.3)^2/2
I2 = (9)(0.2)^2/2
conserving angular momentum,
9* 0.3^2*(20) + 9*(0.2^2) (-20) = {(9)(0.3)^2/2 + (9)(0.2)^2/2 }*w,
solving i t,
w = 15.3846 rev/s
and it rotates in clockwise direction