QUESTION What happens if the woman now leans backwards? (Select all that apply.) Her side of the board moves upward because the average location of her mass is farther from the fulcrum. It depends on whether she leans by moving her feet forward to balance. Her side of the board moves downward if she keeps legs and feet stationary but grasps the board with her hands to balance. Her side of the board moves upward because she is closer to being horizontal. Her side of the board moves upward because her weight produces a smaller torque. Nothing, because her weight still acts at the same location on the board. PRACTICE IT Use the worked example above to help you solve this problem. A woman of mass m = 50.9 kg sits on the left end of a seesaw—a plank of length L = 3.61 m, pivoted in the middle as shown in the figure. (a) First compute the torques on the seesaw about an axis that passes through the pivot point. Where should a man of mass M = 68.6 kg sit if the system (seesaw plus man and woman) is to be balanced? m (b) Find the normal force exerted by the pivot if the plank has a mass of mpl = 10.8 kg. N (c) Repeat part (a), but this time compute the torques about an axis through the left end of the plank. m

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QUESTION What happens if the woman now leans backwards? (Select all that apply.)

Her side of the board moves upward because the average location of her mass is farther from the fulcrum.
It depends on whether she leans by moving her feet forward to balance.
Her side of the board moves downward if she keeps legs and feet stationary but grasps the board with her hands to balance.
Her side of the board moves upward because she is closer to being horizontal.
Her side of the board moves upward because her weight produces a smaller torque.
Nothing, because her weight still acts at the same location on the board.

PRACTICE IT

Use the worked example above to help you solve this problem. A woman of mass m = 50.9 kg sits on the left end of a seesaw—a plank of length L = 3.61 m, pivoted in the middle as shown in the figure.

(a) First compute the torques on the seesaw about an axis that passes through the pivot point. Where should a man of mass M = 68.6 kg sit if the system (seesaw plus man and woman) is to be balanced? m

(b) Find the normal force exerted by the pivot if the plank has a mass of m_pl = 10.8 kg.
N

(c) Repeat part (a), but this time compute the torques about an axis through the left end of the plank.
m

EXERCISEHINTS:

Suppose a 29.8-kg child sits 0.65 m to the left of center on the same seesaw as the problem you just solved in the PRACTICE IT section. A second child sits at the end on the opposite side, and the system is balanced.

(a) Find the mass of the second child.

(b) Find the normal force acting at the pivot point.

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