# Homework Help: Mechanical Equilibrium-torque

1. May 25, 2010

### pat666

1. The problem statement, all variables and given/known data
A metre stick of negligible mass has 1 .2kg mass at its 35cm mark and a .4kg mass at its 75cm mark. where will the fulcrum be if it is balanced?

2. Relevant equations
net torque = 0

3. The attempt at a solution
ive done more complicated questions then this but for some reason i cant figure this one out (logically its more complicated..). I don't know what to do since the radius of revolution will be different for both masses.
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. May 25, 2010

### ehild

Let be the fulcrum at mark "x" and write the torque of both weights with respect to the x mark.

ehild

3. May 25, 2010

### pat666

i tried that but that keeps making the torque radius the same for both masses and it will never be in equilibrium
.2x+.4x=0 x=0 left gravity out because it just cancels

4. May 25, 2010

### pat666

?????? im sure im missing something simple but i really cant think of it

5. May 25, 2010

### ehild

What is torque at all? Is not it the product of force and the distance of the force line from the rotation axis?

ehild

6. May 25, 2010

### pat666

yes thats what it is by definition but to be in equilibrium there has to be two different rotation radii since the masses are not equal. I cannot figure out what to do because of this???????????????????

7. May 25, 2010

### ehild

make drawing.

ehild

8. May 25, 2010

### pat666

i have it didnt help me

9. May 25, 2010

### ehild

Show your drawing. Without it, I do not understand what is in your mind.

ehild

10. May 26, 2010

### pat666

the fulcrum goes at some unknown place between them. didnt bother labeling it because its such a simple drawing.

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11. May 26, 2010

### ehild

Show all the forces and their arms.

ehild

12. May 26, 2010

### arielthw

you can draw it!

http://www.convertmpgtowmv.com" [Broken].

Last edited by a moderator: May 4, 2017
13. May 26, 2010

### pat666

who can draw it-- wtf are you talking about??

14. May 26, 2010

### ehild

I gave up. I wanted a drawing something like this. I made it with Paint in 2 minutes.
d1=35 cm, d2= 75 cm, F1 = 1.2 g, F2=0.4 g, the pivot is at mark x.
What are the conditions of equilibrium?
How do you calculate the torque of a force F with respect to a point, say with respect to the left edge of the metre stick?

ehild

Last edited: Jun 29, 2010
15. May 26, 2010

### xxChrisxx

this is a statics problem. What should you do with any statically determinate statics problem?

Free Body Diagram

Sized for effect. Seriously, get used to drawing them now OP, they are so useful.

16. May 26, 2010

### pat666

net torque and force are 0, still i end up with two variable to solve for though

17. May 26, 2010

### xxChrisxx

You also have 2 equations.

18. May 26, 2010

### pat666

what are my two equations, i have T=rFsin(90) and T=Ia but i dont think i can use that

19. May 26, 2010

### xxChrisxx

Have you drawn a free body diagram?

There are two equations:

Sum of vertical forces.
Sum of moments.

You should have 2 unknowns, so those 2 equations are enough to solve the problem.