A long thin rod of mass M = 2:00 kg and length L = 75:0 cm is free to rotate about its
center as shown. Two identical masses (each of mass m = .403 kg) slide
without friction along the rod. The two masses begin at the rod's point of rotation when
the rod is rotating at 10.0 rad/s.
(a) When they have moved halfway to the end of the rod, how fast (rad/s) is the rod
(b) When the masses are halfway to the end of the rod, what is the ratio of the nal
kinetic energy to the initial kinetic energy (Kf=Ki)?
(c) When they reach the end, how fast is the rod rotating (rad/s)?
Physics - drwls, Sunday, December 25, 2011 at 8:52am
Use conservation of angular momentum.
I*w = constant
I is the moment of inertia, which is
(1/12)Mrod*L^2 + 2m*R^2
R is the distance of the masses from the center of the rod.
w = 10.0 when R = 0
For (a) and (b), R = L/4
For (c), R = L/2
Let's do (a)
Angular momentum with massesw m at R=0:
= 9.38*10^-1 kg m^2/s
(This remains constant).
When R = L/4 = 0.1875 m,
I*w = (2/12)*(0.75)^2*w + 2*(0.403)*(0.1875)^2*w
= (9.38*10^-2 + 2.83*10^-2)w
= 1.221*10^-1*w = 9.38*10^-1
w = 7.68 m/s
For (b), compare initial and final values of (1/2) I w^2
For (c), repeat the process of (a), but use R = L/2