Where Should the Pivot Point Be Placed to Balance a Seesaw?

[answerseeker]

A 70kg adult sits at one end of a 10m board, on other end sits a 30kg child. where should pivot point be placed so board is balanced? board's mass is 15kg.

so far, i have pivot point is 15kg(9.8)=147N (5) <-- half the board length.. for adult i have 686N and kid is 294N. i know that Etorqueccw=Etorquecw ... but I am not sure how to use that for the lengths of both humans, since there are 2 variables. need two eqns then rite? but what's the other eqn?..

 

[Tide]

You (should) have one equation and one unknown. Basically, suppose x is the distance from the pivot point the child. Then the distance to the adult from the pivot point is L - x where L is the length of the board. Put it into your torque equation and solve for x.