Results 1 to 8 of 8

Math Help - calculating moments (torque)?

  1. #1
    Super Member
    Joined
    Sep 2008
    Posts
    607

    calculating moments (torque)?

    Can someone please show me how to do this question, thank you

    the diagram shows a balance beam with two gymnasts standing on it, labelled A and B. The balance beam has a mass of 40kg and is assumed to be uniform. Gymnast A weighs 35kg and gymnast B weighs 30kg. Determine the reaction at each support.
    I have drawn the picture from my book on paint, hopeing somebody can make sense of my picture.
    Attached Thumbnails Attached Thumbnails calculating moments (torque)?-untitled.jpg  
    Last edited by Tweety; November 28th 2008 at 05:06 AM. Reason: Added 'torque' in the heading.
    Follow Math Help Forum on Facebook and Google+

  2. #2
    Super Member
    Joined
    Jun 2008
    Posts
    792
    Take advantage of the fact that a force acting on the pivot produces zero torque.

    Let the reaction forces be labelled R_1, R_2 and they are acting upwards. The system is assumed to be in equilibrium, consequently

    \tau_{\text{net}}=0

    Pivot at R_1, and then find R_2 using the equation:

    \tau_{\text{clockwise}}=\tau_{\text{anti-clockwise}}

    After finding R_2, pivot at R_2 and then find R_1 using the same equation:

    \tau_{\text{clockwise}}=\tau_{\text{anti-clockwise}}
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Super Member
    Joined
    Sep 2008
    Posts
    607
    Quote Originally Posted by Chop Suey View Post
    Take advantage of the fact that a force acting on the pivot produces zero torque.

    Let the reaction forces be labelled R_1, R_2 and they are acting upwards. The system is assumed to be in equilibrium, consequently

    \tau_{\text{net}}=0

    Pivot at R_1, and then find R_2 using the equation:

    \tau_{\text{clockwise}}=\tau_{\text{anti-clockwise}}

    After finding R_2, pivot at R_2 and then find R_1 using the same equation:

    \tau_{\text{clockwise}}=\tau_{\text{anti-clockwise}}
    I dont understand what you saying? Can you like start me off ?
    Last edited by Tweety; November 28th 2008 at 09:53 AM.
    Follow Math Help Forum on Facebook and Google+

  4. #4
    Super Member
    Joined
    Sep 2008
    Posts
    607
    This is what I have done so far, but I don't get the right answer.

    taking moments about R1

     3R_{2} + 35 \times 1= 1.5 \times  40 + 4 \times 30

    can someone show me whats wrong with my equation?

    thank you!

    btw I redrew my picture
    Attached Thumbnails Attached Thumbnails calculating moments (torque)?-untitled.jpg  
    Follow Math Help Forum on Facebook and Google+

  5. #5
    Rhymes with Orange Chris L T521's Avatar
    Joined
    May 2008
    From
    Chicago, IL
    Posts
    2,844
    Thanks
    3
    Quote Originally Posted by Tweety View Post
    This is what I have done so far, but I don't get the right answer.

    taking moments about p1

     3R_{2} + 35*1= 1.5 * 40 + {\color{red}1}* 30

    can someone show me whats wrong with my equation?

    thank you!

    btw I redrew my picture
    Everything looks right (assuming that the LHS are the counterclockwise torques, and the RHS are clockwise)

    However, the number in red should be 4, because the 30kg mass is 4 meters away from the pivot (p1) [Do you see why?].
    Follow Math Help Forum on Facebook and Google+

  6. #6
    Super Member
    Joined
    Sep 2008
    Posts
    607
    Quote Originally Posted by Chris L T521 View Post
    Everything looks right (assuming that the LHS are the counterclockwise torques, and the RHS are clockwise)

    However, the number in red should be 4, because the 30kg mass is 4 meters away from the pivot (p1) [Do you see why?].


    oh yes sorry that was a typo, I meant to put 4 meters.

    my equation can't be right cause I don't get the right answers, the reactions forces R1 and R2 equal 474N and 555N. thats the answers given by my book ?
    Follow Math Help Forum on Facebook and Google+

  7. #7
    Rhymes with Orange Chris L T521's Avatar
    Joined
    May 2008
    From
    Chicago, IL
    Posts
    2,844
    Thanks
    3
    Quote Originally Posted by Tweety View Post
    oh yes sorry that was a typo, I meant to put 4 meters.

    my equation can't be right cause I don't get the right answers, the reactions forces R1 and R2 equal 474N and 555N. thats the answers given by my book ?
    Ah...that's it! You're answer is in kilograms, not newtons. To fix that, multiply your answers by 9.8 and you should get the desired answers.
    Follow Math Help Forum on Facebook and Google+

  8. #8
    Super Member
    Joined
    Sep 2008
    Posts
    607
    Quote Originally Posted by Chris L T521 View Post
    Ah...that's it! You're answer is in kilograms, not newtons. To fix that, multiply your answers by 9.8 and you should get the desired answers.
    Oh right!! I dont believe I missed that! thanks a lot for you help.
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Torque question
    Posted in the Math Topics Forum
    Replies: 1
    Last Post: May 4th 2010, 04:45 PM
  2. mechanics help, torque
    Posted in the Algebra Forum
    Replies: 3
    Last Post: November 29th 2008, 09:10 AM
  3. Calculating the Moments
    Posted in the Calculus Forum
    Replies: 0
    Last Post: November 19th 2008, 02:37 PM
  4. Magnitude of Torque
    Posted in the Calculus Forum
    Replies: 4
    Last Post: September 6th 2008, 02:43 PM
  5. Torque problem
    Posted in the Math Topics Forum
    Replies: 1
    Last Post: November 30th 2007, 09:32 AM

Search Tags


/mathhelpforum @mathhelpforum