Substitute all known quantities. Solve for x substituting the normal force found in part (b). Exercise 8.3 -(78.3 kg)(9.80 m/s)(1.85 m + x) + (54.6 kg)(9.80 m/s)(0) - (11.5 kg)(9.80 m/s)(1.85 m) + n(1.85 m) = 0 -(1.42 x 10 Nm) - (767.34)x - (208.495 Nm) + (1.85 m)n = 0 X = Remarks The answers for x in parts (a) and (c) agree except for a small round-off discrepancy. This illustrates how choosing a different axis leads to the same solution. Hints: Getting Started | I'm Stuck Suppose a 31.2 kg child sits 1.50 m to the left of center on the same seesaw. A second child sits at the end on the opposite side, and the system is balanced. m = (a) Find the mass of the second child. mchild 2 = X kg Use the pivot as the reference point because then two of the four forces on the plank exert zero torque. Which two? Since the plank is at rest, the torques due to the other two forces must balance (b) Find the normal force acting at the pivot point. Fn XN Your expression is correct, but since your answer to part (a) is wrong, this answer is wrong too.