Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Mechanical Equilibrium-torque

  1. May 25, 2010 #1
    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. jcsd
  3. May 25, 2010 #2

    ehild

    User Avatar
    Homework Helper

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

    ehild
     
  4. May 25, 2010 #3
    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
     
  5. May 25, 2010 #4
    ?????? im sure im missing something simple but i really cant think of it
     
  6. May 25, 2010 #5

    ehild

    User Avatar
    Homework Helper

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

    ehild
     
  7. May 25, 2010 #6
    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???????????????????
     
  8. May 25, 2010 #7

    ehild

    User Avatar
    Homework Helper

    make drawing.

    ehild
     
  9. May 25, 2010 #8
    i have it didnt help me
     
  10. May 25, 2010 #9

    ehild

    User Avatar
    Homework Helper

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

    ehild
     
  11. May 26, 2010 #10
    the fulcrum goes at some unknown place between them. didnt bother labeling it because its such a simple drawing.
     

    Attached Files:

  12. May 26, 2010 #11

    ehild

    User Avatar
    Homework Helper

    Show all the forces and their arms.

    ehild
     
  13. May 26, 2010 #12
    you can draw it!


    http://www.convertmpgtowmv.com" [Broken].
     
    Last edited by a moderator: May 4, 2017
  14. May 26, 2010 #13
    who can draw it-- wtf are you talking about??
     
  15. May 26, 2010 #14

    ehild

    User Avatar
    Homework Helper

    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
  16. May 26, 2010 #15
    this is a statics problem. What should you do with any statically determinate statics problem?

    Answer:

    Free Body Diagram

    Sized for effect. Seriously, get used to drawing them now OP, they are so useful.
     
  17. May 26, 2010 #16
    net torque and force are 0, still i end up with two variable to solve for though
     
  18. May 26, 2010 #17
    You also have 2 equations.
     
  19. May 26, 2010 #18
    what are my two equations, i have T=rFsin(90) and T=Ia but i dont think i can use that
     
  20. May 26, 2010 #19
    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.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook