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S E C T I O N 1 1 . 4 • Conservation of Angular Momentum 345 Hence, the net torque exerted on the system about O is To find *, we use + ext # I*, where I was obtained in part (A): Generally, fathers are more massive than daughters, so the 2(m f $ m d )g cos " " $ M 3 % m f % m d % * # # + ext I # # # + ext # + f % + d # 1 2 (m f $ m d )g " cos " What If? After several complaints from the daughter that she simply rises into the air rather than moving up and down " ? Answer The angular acceleration of the system should de- The total moment of inertia about the z axis through O for the modified system is The net torque exerted on the system about O is Now, the angular acceleration of the system is The seesaw will be balanced when the angular acceleration In the rare case that the father and daughter have the same d # $ m d m f %
1 2 " m f gd cos " $ 1 2 m d g " cos " # 0 * # m f gd cos " $ 1 2 m d g " cos " (" 2 /4)[(M/3) % m d ] % m f
d 2 # 0 * # + net I # m f gd cos " $ 1 2 m d g " cos " " 2 4 [(M/3) % m d ] % m f
d 2 + net # + f % + d # m f gd cos " $ 1 2 m d g " cos " # " 2 4
$ M 3 % m d % % m f
d 2 I # 1 12 M" 2 % m f d 2 % m d
$ " % 2 Figure 11.9 (Example 11.6) A father and daughter demon- strate angular momentum on a seesaw. At the Interactive Worked Example link at http://www.pse6.com, you can move the father and daughter to see the effect on " O θ y x m d g m f g 11.4 Conservation of Angular Momentum In Chapter 9 we found that the total linear momentum of a system of particles remains The total angular momentum of a system is constant in both magnitude and direc- Conservation of angular momentum |