The figure shows a rigid assembly of a thin hoop (of mass m = 0.19 kg and radius

Question

The figure shows a rigid assembly of a thin hoop (of mass m = 0.19 kg and radius R = 0.14 m) and a thin radial rod (of length L-2R and also of mass m = 0.19 kg). The assembly is upright, but we nudge it so that it rotates around a horizontal axis in the plane of the rod and hoop, through the lower end of the rod. Assuming that the energy given to the assembly in the nudge is negligible, what is the assembly's angular speed about the rotation axis when it passes through the upside-down inverted) orientation? Hoop Rod Rotation axis Number Units

Explanation / Answer

rcm = location of the center of mass = (mh rh + mL rL )/(mh + mL) = ((0.19) (3R) + (0.19) (R))/(0.19 + 0.19)

rcm = 2R = 2 x 0.14 = 0.28 m

Ih = moment of inertia of the hoop = (0.5) mR2 + m(3R)2 = (9.5) m R2

IR = moment of inertia of the Rod = m(2R)2/3 = 1.33 m R2

using conservation of energy

Potential energy = rotational KE

(2m) g rcm = (0.5) (Ih + Ir) w2

(2 x 0.19) (9.8) (0.28) = (0.5) (1.33) (0.19) (0.14)2 w2

w = 20.5 rad/s

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