10.69. Set Up:The free-body diagram for the drawbridge is given in Figure 10.69. For an axis at the lower end,Figure 10.69Solve: (a)gives and(b)depends on the angle the bridge makes with the horizontal. is not constant during the motion andcannot be used.(c)Use conservation of energy. Take at the lower end of the drawbridge, so andgives andReflect:If we incorrectly assume that is constant and has the value calculated in part (a), then gives The angular acceleration increases as the bridge rotates and the actual angularvelocity is larger than this.10.70. Set Up:The free-body diagram for the leg is given in Figure 10.70a. Let the pivot be at the hip joint andtake counterclockwise torques to be positive. There is a force of unknown magnitude and direction that the hipjoint exerts on the leg, but this force produces zero torque.Figure 10.70Solve: (a)gives and (b)
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