Stability & Balance of rigid bodies

1. Jan 16, 2010

M_G

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

A steel girder with a mass of 1000kg and a length of 10m rests on a concrete slab, with 4m overhanging the edge. How far can a 100kg man walk on the girder?

3. The attempt at a solution

I supposed that the right end of the slab is the fulcrum (rotating point), and according to the law of equilibrium of rigid bodies, the sum of torques around that point will be zero,
If the maximum distance the person could walk is X, then the eqn. will be:
100kg * X m = 1000Kg * 1 m (segma tao = 0)
Where:
1 m is the distance between the point of center of gravity of the steel girder (length/2 m)& the fulcrum.....

But.....X=10m ! I missed something, but I don't know what? am I proceeding right from the begining??

2. Jan 16, 2010

tiny-tim

Hi M_G!
uhh? where will the girder actually rotate around if the man is too heavy?

3. Jan 16, 2010

M_G

you mean it will not rotate? so what will happen when this person walk on it?
topple on the other side, right?

4. Jan 16, 2010

tiny-tim

Of course it will rotate …

I asked where it will rotate round.​

5. Jan 17, 2010

M_G

ok..but there will be a torque..?

Last edited: Jan 17, 2010
6. Jan 17, 2010

tiny-tim

uhh? there's always torque.

What point does the girder rotate around?

7. Jan 17, 2010

M_G

I'm sorry...I'm confused a little bit, I'll try to think about it again...

8. Jan 17, 2010

M_G

The right end of slab?

9. Jan 17, 2010

tiny-tim

oh sorry, i've just realised i've misread the original question

let me start again …
Yes, your method of torques is correct …

if the man could walk 10m, the girder would still just balance …

but he can't …

so the answer is that he can walk the full 4m.

10. Jan 17, 2010

Thanx!