# Moments problem

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1. Jul 17, 2016

### IhasSwag

1. The problem statement, all variables and given/known data
The bar has a mass of 3 grams/cm. When a mass of 15 grams is hung on the left side, the system is balanced. Calculate the length of the bar.

2. Relevant equations
F1*r1 = F2*r2

3. The attempt at a solution
I thought that the moment on the left should be equal to the moment at the right, so it should be
mleft * g * rleft = mright* g *rright

But apparently it should be that the moment of the mass on the left side is equal to the moment of the whole bar.
Can someone explain why this is?

2. Jul 17, 2016

### giokrutoi

what is the mass of the right side

3. Jul 17, 2016

### IhasSwag

the bar is 15cm on the right side. The mass is 3 grams / cm. So 45 grams.

4. Jul 17, 2016

### giokrutoi

the center of mass on bar where is it ?

5. Jul 17, 2016

### giokrutoi

and lenght is not 15
it is 15 + x

6. Jul 17, 2016

### IhasSwag

isn't the center of mass exactly in the middle?

7. Jul 17, 2016

### giokrutoi

yes an it is in 15 + x /2

8. Jul 17, 2016

### giokrutoi

so what is distance from center of mass to the thing that balances the bar

9. Jul 17, 2016

### IhasSwag

I really don't know

10. Jul 17, 2016

### giokrutoi

look on the picture the center of mass is exactly in the middle and i am asking you to tell what is distance between the center and the big black thing that is holing the bar

11. Jul 17, 2016

### IhasSwag

is it: 15 - (15+x)/2 ?

12. Jul 17, 2016

### giokrutoi

no but you were close (15 + x ) / 2 - x
edit : x doesn't belong to 2

13. Jul 17, 2016

### IhasSwag

I see now that it is (15 + x ) / 2 - x

14. Jul 17, 2016

### giokrutoi

if you got it than what can you say about forces

15. Jul 17, 2016

### IhasSwag

They cancel out at the center of mass?

16. Jul 17, 2016

### giokrutoi

nope what do you think is holding 15 grams from not falling

17. Jul 17, 2016

### IhasSwag

The mass of the bar on the right side?

18. Jul 17, 2016

### giokrutoi

what do you mean under mass on the right side do you say that it is smaller that hole mass of bar

19. Jul 17, 2016

### IhasSwag

I meant the mass of the bar on the right side of the center of mass

20. Jul 17, 2016

### giokrutoi

that is not correct in center of mall is localized hole mass so it fully acts on the "holder"