# Mass of a child on a seesaw

1. Oct 11, 2008

### jessica.so

1. The problem statement, all variables and given/known data
Two children balance on a seesaw. One, with a mass of 30 kg, is seated 2.0 m from the pivot. The other is seated 2.5 m from the pivot. What is the mass of the second child?

2. Relevant equations
N/A

3. The attempt at a solution
I drew the diagram of the two children and I found the force of gravity acting on the first child:
Fg=mg ; Fg=(30 kg)(9.8 m/s/s) ; Fg = 294N

I know that the Net Force would be 0 because the seesaw is balanced.
And then...I dont know what to do...

2. Oct 11, 2008

### Rake-MC

Okay, so they both have the same acceleration acting on them $$g = 9.81ms^{-1}$$

However they both have torques created by the distance from the pivot point.

We know that $$\tau = Fr$$

So we're saying that $$(30)(9.8)(2.0) = (m)(9.8)(2.5)$$

3. Oct 11, 2008

### jessica.so

ohh okay! and then you would just isolate and...yaa! Thanks so much for your help!! I really really appreciate it!!!!