Two disks are located on an axle as shown in the figure below. The lower disk is

Question

Two disks are located on an axle as shown in the figure below. The lower disk is initially spinning at 51 rad/s, and the upper one is not spinning. The upper disk then falls onto the lower disk, and they stick together. Is there a torque on the lower disk? On the upper disk? Yes No (b) Identify a system for which the external torque is zero so that the angular momentum is conserved. The external torque is zero for a system comprised of both disks (c) What is the initial angular momentum of your system? kg.m2/s (d) What is the final angular momentum of your system? Express your answer in terms of w, the final angular velocity of the two disks. (Recall that w has units of rad/s and angular momentum will have units of kg . m2.) (e) Find Wf.

Explanation / Answer

c) Li = Ii*w1

= (0.5*m1*R1^2)*w1

= (0.5*20*2^2)*51

= 2040 kg.m^2/s

d) Lf = If*wf

= (0.5*m1*R1^2 + 0.5*m2*R2^2)*wf

= (0.5*20*2^2 + 0.5*7*1^2)*wf

= 43.5*wf

e) Li = Lf

2040 = 43.5*wf

==> wf = 2040/43.5

= 46.9 rad/s

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