Simple Static Equilibrium question

[Femme_physics]

Homework Statement

Two workers carry a beam at the length of 3 meters. One worker holds the beam at the edge and carries 0.4 of the beam's weight. At what distance (x) from the other edge holds the second worker?

Homework Equations

None.

The Attempt at a Solution

The solution is x = 0.5 [m]

I just don't know the way... can anyone help me understand this?

 

[tiny-tim]

Hi Dory!

Two workers carry a beam at the length of 3 meters. One worker holds the beam at the edge and carries 0.4 of the beam's weight. At what distance (x) from the other edge holds the second worker?

Hint: there are three forces on the beam …

the weight operating through the centre of mass, and the two forces from the workers.

 

[Femme_physics]

Hi Dory!


Hint: there are three forces on the beam …

the weight operating through the centre of mass, and the two forces from the workers.

Hi!

There are 3 forces, yes, mg, and the 2 workers. I've tried calculating the sum of all moments on all 3 points but something isn't right in my answers, and I'm not even sure if I'm using the right equations. I'm not used to these questions coming without diagrams...

 

[tiny-tim]

Take moments about anywhere, and it should work …

show us what you get. ​

 

[pongo38]

Now is your chance to do a diagram of your own and which demonstarates YOUR understanding of this problem.

 

[Femme_physics]

Sum of all moments on G (center of gravity) = 0.4mg x 1.5 -0.6mg x (unknown length)

If I do that the result is 1...? That can't be the answer...it says the answer is 0.5 [m]...

As for the free body diagram...there... arg...still can't see how...darn it, I solved muuuuuuuuuch harder problems than this...that's annoying I can't figure it out!


http://img405.imageshack.us/img405/5662/thereh.jpg

 

[nvn]

Dory: Nice work. Your result is correct, so far. Now, the question asks you for the distance, x, measured from the left-hand edge. Can you think of how to compute it?

 

[Femme_physics]

Well, there are 2 distances here really. 3 meter (distance from worker to worker) and 1.5 meter, distance from worker to center of gravity. Do you see any more distances?

 

[pongo38]

You have got the 0.6 at 3m from the 0.4, but it should be x from there. This is a moments question, but first, get your figure right.

 

[tiny-tim]

Hi Dory!

(just got up :zzz: …)

… At what distance (x) from the other edge holds the second worker?

Sum of all moments on G (center of gravity) = 0.4mg x 1.5 -0.6mg x (unknown length)

If I do that the result is 1...? That can't be the answer...it says the answer is 0.5 [m]...

D'oh!

That is right! You got 1 from the centre, which is 0.5 from the edge!

Always read the question carefully!

btw, sorry but your diagram is really bad …

you haven't marked "x" on it at all, and you've put both the forces at the ends.

A clear diagram is essential for doing these questions (although you don't get any marks for the diagram in an exam, you certainly lose marks if you make mistakes because your diagram is bad, and you can also lose time )​

 

[Femme_physics]

Oh, so I made a mistake in the diagram, they're holding it some distance AWAY from the edge... ah...got it, thanks :D

Thanks for leading me up to it!