Disk A, with a mass of 2.0 kg and a radius of 50 cm , rotates clockwise about a
ID: 1600713 • Letter: D
Question
Disk A, with a mass of 2.0 kg and a radius of 50 cm , rotates clockwise about a frictionless vertical axle at 20 rev/s . Disk B, also 2.0 kg but with a radius of 40 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. Part A After the collision, what is magnitude of their common angular velocity (in rev/s)? Express your answer using two significant figures. In which direction do discs rotate after the collision?
Explanation / Answer
I1 = (2)(0.5)^2/2=0.25
I2 = (2)(0.4)^2/2 = 0.16
1 = 20 rev/s = 30*2*pi rad/s = 188.4 rad/s
2 = -20 rev/s = -30*2*pi rad/s = -188.4 rad/s
I11 + I22 = (I1 + I2)
0.25*188.4 + 0.16*-188.4 =(0.25+ 0.16 )
= -26.42 rev/s
The composite body will rotate counterclockwise at -26.42 = -4.21 rev/s