Question

PRACTICE IT Use the worked example above to help you solve this problem. A woman of mass m58.7 kg sits on the left end of a seesaw-a plank of length L = 4.61 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 69.9 kg sit if the system (seesaw plus man and woman) is to be balanced? 1.936 (b) Find the normal force exerted by the pivot if the plank has a mass of mpl 12.6 kg. 1383 IN (c) Repeat part (a), but this time compute the torques about an axis through the left end of the plank.

Answer 1

a) torque about pivot = mgL/2 = 58.7 x 9.8 x 4.61/2 = 1325.9743 Nm

Man should sit to right of pivot at distance = 1325.9743/(69.9×9.8) = 1.936 m

b) normal force = (12.6 + 69.9 + 58.7)9.8 = 1383.76 N

c) making torque about left = 0

=> 1383.76 x 4.61/2 = 12.6 x 9.8 x 4.61/2 + 69.9 x 9.8 x (4.61/2 + x)

=> x = 1.936 m