To solve the problem it is necessary to apply the concepts of Torque and equilibrium.
The Torque is defined as:
Where
F= Force
d = Distance
In this particular case, the force is caused by the weight of both children.
In turn, when there is equilibrium, the two torques must be equal therefore
Replacing with our values
Re-arrange to find
Therefore the distance that the second kid should sit to balance the see-saw is 1.8m from the pivot.
