Disk A, with a mass of 2.0 kg and a radius of 90 cm , rotates clockwise about a
ID: 1794080 • Letter: D
Question
Disk A, with a mass of 2.0 kg and a radius of 90 cm , rotates clockwise about a frictionless vertical axle at 20 rev/s . Disk B, also 2.0 kg but with a radius of 70 cm , rotates counterclockwise about that same axle, but at a greater height than disk A, at 20 rev/s . Disk B slides down the axle until it lands on top of disk A, after which they rotate together.
A) After the collision, what is magnitude of their common angular velocity (in rev/s)?
B) In which direction do disks rotate after the collision? clockwise or counterclockwise
Thanks!
Explanation / Answer
IA = M R^2 /2 = 2 x 0.90^2 / 2 = 0.810 kg m^2
IB = 2 x 0.70^2 /2 = 0.490 kg m^2
wA = - 20 rev/s
wB = + 20 rev/s
(A) Applying angular momentum conservation,
IA wA + IB wB = (IA + IB) wf
wf = - 4.92 rev/s
angular speed = 4.92 rev/s ........Ans
(B) direction - clockwise