1. Aug 10, 2011

### danief

1. The problem statement, all variables and given/known data

http://i.imgur.com/CnAcu.png

2. Relevant equations

So, I'm having trouble figuring out how to start this problem.

I'm trying to use this equation:

Fe = W(1+$\sqrt{1+2h/\deltast}$

and $\delta$st = W/k

3. The attempt at a solution

This is my attempt at summing the forces:

$\Sigma$F = 100 lbf/in * $\delta$ + Weight of child

Thanks

Last edited by a moderator: Aug 14, 2011
2. Aug 12, 2011

### PhanthomJay

You should first calculate the spring deformation under 400 pounds of max force in the spring, using Hooke's law. Then use conservation of energy equation.

Last edited by a moderator: Aug 14, 2011
3. Aug 14, 2011

### nvn

danief: Your relevant equations are correct; and they are not a function of maximum deflection. You can change your approach as PhanthomJay suggests; or you can instead just use the approach you already started writing in your relevant equations, as follows.

First, convert everything to a coherent measurement system. Next, the problem statement gives you h = 0.1016 m, Fe = 1779.2886 N, and spring constant k. Now simply solve your relevant equation for weight W. You are done.

(By the way, ignore your attempt at summing forces in part 3 of post 1, which is incorrect.) Also, please do not post wide images directly to the forum page. Just post a text link to wide images.