- #1
Niles
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Homework Statement
I have to find the kinetic energy of the total system; b has mass 2m and a has mass m. They rotate around the y-axis and and through the origin (0,0).
There's two ways of finding the kinetic energy of the system around the y-axis.
1) I find moment of inertia for each particle around the y-axis (and through the origin) and add - so it's 2*m*r^2+9*m*r^2 = 11*m*r^2. Then I use this in K = ½*I*w^2.
2) I find the center of mass: r_cm = (2*m*r+m*3*r)/(3m) = (5/3)r. I use this in I = m*r^2 and then use K = ½*I*w^2.
#2 doesn't work - why?