physics torque problems?

Question

a 60 kg boy sits 1.8 meters from the pivot point (left end) on a teeter totter which has a mass of 15 kg. the tetter totter is 4 meters long. Where must a 49 Kg girl sit in order for the teeter totter to balance.

Answer 1

Sum the torques about the pivot pooint

60*1.8 for the boy
.9*15/4 for the teeter totter on the boy's side

-15*(4-1.8)/2 for the teeter totter on the boy's side
-49*d for the girl, where d is the distance from the balance point
sum and set to 0

60*1.8 +.9*15/4
=15*(4-1.8)/2-49*d
d=(60*1.8 +(.9*15/4)-(15*(4-1.8)/2))/49
d=1.94 m
this is less than the 2.2 m of teeter totter she has on her side, so it satisfies that constraint.

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