1. Limited time only! Sign up for a free 30min personal 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!

Moment about a point?

  1. Jan 25, 2012 #1
    Calculate the resultant of the four forces shown and their total moment about A. Hence determine the perpendicular distance from A to the line of the single force which would have the same effect as the forces F1, F2, F3 and F4 combined.
    Assume the following values:
    F1 = 55N, F2 = 33N, F3 = 74N and F4 = 68N
    Note that, in this diagram, angles shown are not drawn accurately



    Could you please give me some clue of how to answer this question? my main problem is that I don't know how to find the distance between A and each forces, I'm so confused.
    Thanks for your help.
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     

    Attached Files:

  2. jcsd
  3. Jan 25, 2012 #2

    PhanthomJay

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    If you break up each of the forces into their x and y components, the perpendicular distances from the line of action of the forces components to point A will become obvious. Then the moment about A of all forces can be obtained by summing all moments of each component force. Watch plus and minus signs.
     
  4. Jan 25, 2012 #3
    PhanthomJay,Thank you very much,Considering what you said I used this formula to find each force and then The total force:

    F * (sin or cos) * perpendicular distance = Moment


    F1 * sin90 * 2 = 110
    F2 * sin78 * 5 = 161.39
    F3 * sin42 * 3 = 148.54
    -(F4) * cos77 * 2 = -30.59

    the total force = 389.34

    If this is right then how can I find the prependicular distance for the total force? or how can I draw the total force on the picture?
     
  5. Jan 25, 2012 #4

    PhanthomJay

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    These values are the correct moments of the y components of the forces about A. You need to calculate the moments of the x components also.
    This is the total moment about A from the y component of forces. You need to determne the total moments from the x component of the forces also.
    The sum total of the y components of forces is the y component of the resultant single force, Ry. The sum of the total x components of forces is the x component of the resultant single force, Rx. If you divide the total moment of the y component forces by the total y component forces, you get the perp distance from the line of action of Ry to A. If you divide the total moment of the x component forces by the total x component forces, you get the perp distance from the line of action of Rx to A. The magnitude and direction of R is determined from pythagoras' theorem and trig, and it passes through the coordinate where Rx and Ry meet.
     
  6. Jan 26, 2012 #5
    PhanthomJay , You are brilliant, Thank you so much, couldn't be explained better than that.
    wonderful.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Moment about a point?
Loading...