Use the worked example above to help you solve this problem. A woman of mass m =

Question

Use the worked example above to help you solve this problem. A woman of mass m = 54.9 kg sits on the left end of a seesaw-a plank of length L = 3.79 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 = 74.9 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 = 10.2 kg. ______ N (c) Repeat part (a), but this time compute the torques about an axis through the left end of the plank. _______ m Suppose a 29.9-kg child sits 1.14 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. _______ kg (b) Find the normal force acting at the pivot point. _______ N

Explanation / Answer

(a)

distance freom center to end = r1 = L/2

torque = Fg*r1 = m1*g*r = 54.9*9.8*3.79/2

net torque = 0

m1*g*r1 = m2*g*r

54.9*9.8*3.79/2 = 74.9*9.8*r

r = 1.38 m   from the axis <<<---------answer

(b)

along vertical

-Fg1 - Fg2 + N = 0

Normal force N = (m1+m2)*g

N = (54.9+74.9)*9.8

N = 1272.04 N

(c)

N*L/2 = m2*g*r

1272.04*3.79/2 = 74.9*9.8*r

r = 3.28 m     from the left end <<<-------answer

======================================

when the system is balanced

net torque = 0

m1*g*r1 = m*g*L/2

29.9*9.8*1.14 = m*9.8*3.79/2

m = 18 kg <<<_----------answer

(b)

N = m1*g + m2*g

N = (29.9+18)*9.8

N = 469.42 N <<<<=========answer

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