Balancing Teeet Totter: Lever Arm Calculation

[alex7298]

Test Review help!

Homework Statement

Two children whose masses are 35kg and 55kg are on a playground "teeet totter" which is 3.0m long. Where must the fulcrum be placed so that the children will balance?
(35kg on left, 55kg on right)

Homework Equations

sum of T=0

The Attempt at a Solution

I started with T=0=F(left)L(left)-F(right)L(right)
The two lever arms must add to be 3.0m so L(left)=3.0m-L(right). Through some algebra i got the equation into terms of L(right):
L(right)=(3m)(Fleft)/(2(Fright))
I get the right lever to be .95m, but is supposed to be between 1-1.5

Where am i going wrong??
Thanks

 

[alex7298]

Anyone??

 

[rock.freak667]

Start simple.

Let the distance from one end be x metres, then the distance from the fulcrum to the other end is (3-x)metres.

For it to be balances the resultant torque about the fulcrum must be zero.
Can you set up the equation now?

 

[foxjwill]

This is all assuming the teeter totter is massless.

 

[alex7298]

teacher said you can ignore mass of teeter totter.
And to rock.freak667, that's exactly how i started. I got 0=(x-3)(Fleft)-(X)(Fright)...

 

[rock.freak667]

Clockwise moment about pivot=(33gx) Nm
Anti-clockwise moment about pivot=55g(3-x) Nm

For balance

55g(3-x)=33gx


Are you sure you did the algebra correctly?

55(3-x)=33x
165-55x=33x
88x=165

 

[timon]

r*m = r*m