12.68. Model: The bar is a solid body rotating through its center. Visualize:Solve:(a)The two forces form a couple. The net torque on the bar about its center is netILFIFLατα==⇒=where F is the force produced by one of the air jets. We can find Iand as follows: 2222101011(0.50 kg)(0.60 m)0.015 kg m1212()150 rpm5.0 rad/s0 rad(10 s0 s)0.50 rad/s(0.015 kg m )(0.5 rad/s )0.0393 N(0.60 m)IMLttFωωαπ==+ −⇒==+− ⇒=⇒==The force 39 mN.F=(b)The torque of a couple is the same about any point. It is still net.LFτ=However, the moment of inertia has changed. 2
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