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: Physics problem

  1. Aug 6, 2004 #1
    i am having problems figuring out how to resolve the moments can someone help me please

    Determine the reactions at "A" and "B"

    A is a moving support
    B is a fixed support

    please check attached file for problem

    Attached Files:

    • mat.txt
      File size:
      656 bytes
  2. jcsd
  3. Aug 6, 2004 #2

    Tom Mattson

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Got your Private Message. :wink:

    OK, I need to see what you've done on this (that's our policy). It's not a difficult problem, so don't be afraid of it. Take the following steps:

    1. Calculate the moments due to the loads shown. The standard convention is that counterclockwise moments are in the positive direction, and that clockwise moments are in the negative direction.

    So think physically: In which direction will each load tend to rotate the lever about the fulcrum? CW or CCW?

    2. Determine the reactions with Newton's laws. Sum the forces in the y-direction, and sum the moments in the z-direction. Include unknown reaction forces, and set the sums equal to zero. Then solve for the unknowns.
  4. Aug 7, 2004 #3
    If a force is acting upwards or downwards from the point at which you are resolving, it is the horizontal (do not include vertical) distance to the force (so the point to measure to would be exactly above or below the force). multiplied by the force.
    Similarly, if a force is acting horizontally from the point at which you are resolving it is the vertical distance multiplied by the force magnitude.

    If the force is acting at an angle you can resolve everything into x and y components, to make it much easier to calculate.
    For example: For a 10N force acting at 23 degrees, the horizontal force is 10cos23, while the vertical force is 10sin23. From my memory though this doesn't seem to be a problem in your question.

    -For the reaction at B you don't need moments, the forces acting vertically are in balance. From this you can calculate B.
    -For the reaction at A, perhaps it would make it simpler for you to understand if you removed 'A' and replaced it with a horizontal force (the reaction force at A) which you can then calculate. Also, add in the calculated force at B.
    -Then choose a convenient position to resolve moments at. Somewhere where the force at A is not cancelled out perhaps?

    Then take moments,
    Clockwise = anticlockwise.

    Have a go =).
    Last edited: Aug 7, 2004
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook