D. Nothing will happen; the seesaw will still be balanced.
<h3>Further explanation
</h3>
The force acting on a system with static equilibrium is 0
(forces acting as translational motion only, not including rotational forces)
For objects undergoing rotation, the equilibrium must be met
A heavy boy (Hb) and a lightweight girl (Lg) are balanced on a mass-less seesaw
Because there is a balance of rotation, the torque equation:
Στ = 0
Hb.r1-Lg.r2 = 0
Hb.r1 = Lg.r2 (equation 1)
If they both move forward so that they are one-half their original distance from the pivot point, then the distance of the two children to the pivot point is reduced to half
Then the torque equation:
![\rm Hb\times \dfrac{r_1}{2}= Lg\times \dfrac{r_2}{2}\\\\Hb\times r_1=Lg\times r_2](https://tex.z-dn.net/?f=%5Crm%20Hb%5Ctimes%20%5Cdfrac%7Br_1%7D%7B2%7D%3D%20Lg%5Ctimes%20%5Cdfrac%7Br_2%7D%7B2%7D%5C%5C%5C%5CHb%5Ctimes%20r_1%3DLg%5Ctimes%20r_2)
This equation remains the same as equation 1, so the seesaw will still be balanced.
<h3>
Learn more
</h3>
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