Question

Two dlsks are located on an axle as shown In the figure below. The lower disk Is Initlally spinning at 24 rad/s, and the upper one Is not spinning. T upper dlsk then falls onto the lower dlsk, and they stlck together. m2 7.0 kg R1.0 m m 20 kg R1 2.0 m Initial position Final position (a) Is there a torque on the lower disk? Yes No 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 -Select (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 wf, the final angular velocity of the two disks (Recall that wf has units of rad/s and angular momentum will have units of kg m2.) (e) Find of rad/s

Answer 1

a) yes the torque is due to upper disk Yes it's due to lower disk b) The required system is comprised of lower disk and upper disk c) L_i = I_i W_i = (m_1 R_1/2)^2 (24 rad/s) = 960 kg m^2/s d) L_f = (I_1 + I_2)w_f = (m_1 R_1^2/2 + m_2 R_2^2/2)w_f = 43.5 w_f e) L_i = L_f 960 kg.m^2/s = 43.5 w_f or w_f = 22.07 rad/s

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