1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Moments problem

Tags:
  1. Jul 17, 2016 #1
    Moments.png 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. jcsd
  3. Jul 17, 2016 #2
    what is the mass of the right side
     
  4. Jul 17, 2016 #3
    the bar is 15cm on the right side. The mass is 3 grams / cm. So 45 grams.
     
  5. Jul 17, 2016 #4
    the center of mass on bar where is it ?
     
  6. Jul 17, 2016 #5
    and lenght is not 15
    it is 15 + x
     
  7. Jul 17, 2016 #6
    isn't the center of mass exactly in the middle?
     
  8. Jul 17, 2016 #7
    yes an it is in 15 + x /2
     
  9. Jul 17, 2016 #8
    so what is distance from center of mass to the thing that balances the bar
     
  10. Jul 17, 2016 #9
    I really don't know
     
  11. Jul 17, 2016 #10
    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
     
  12. Jul 17, 2016 #11
    is it: 15 - (15+x)/2 ?
     
  13. Jul 17, 2016 #12
    no but you were close (15 + x ) / 2 - x
    edit : x doesn't belong to 2
     
  14. Jul 17, 2016 #13
    I see now that it is (15 + x ) / 2 - x
     
  15. Jul 17, 2016 #14
    if you got it than what can you say about forces
     
  16. Jul 17, 2016 #15
    They cancel out at the center of mass?
     
  17. Jul 17, 2016 #16
    nope what do you think is holding 15 grams from not falling
     
  18. Jul 17, 2016 #17
    The mass of the bar on the right side?
     
  19. Jul 17, 2016 #18
    what do you mean under mass on the right side do you say that it is smaller that hole mass of bar
     
  20. Jul 17, 2016 #19
    I meant the mass of the bar on the right side of the center of mass
     
  21. Jul 17, 2016 #20
    that is not correct in center of mall is localized hole mass so it fully acts on the "holder"
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Moments problem
  1. A problem in moment (Replies: 1)

  2. Moments problem (Replies: 15)

Loading...