Three masses are suspended from a meterstick as shown in Figure 8.8a. How much mass must be suspended on the right side for the system to be in static equilibrium? (Neglect the mass of the meterstick.) 
THINKING IT THROUGH. As the free-body diagram (Figure 8.8b) shows, the translational equilibrium condition will be satisfied with the upward normal force \(\overrightarrow{\mathbf{N}}\) balancing the downward forces caused by the objects weights, so long as the stick remains horizontal. But \(\overrightarrow{\mathbf{N}}\) is not known if \(m_{3}\) is unknown, so applying the condition for rotational equilibrium should give the required value of \(m_{3}\). (Note that the lever arms are measured from the pivot point, the center of the meterstick.)

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25 g 20 cm -12 (a) 75g 1112 50 cm 85 cm 100 cm ? m3