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Find reaction to beam

  1. Feb 10, 2013 #1
    1. The problem statement, all variables and given/known data

    Find reaction for given. one beam 15 ft long supported at end (R1) and supported 5foot in from the ohter end (R2). 2000 lbs load is location at the very end on the beam 5foot from r2

    2. Relevant equations

    how do I determian the reactions? I know how to draw the shear and moment diagrams

    3. The attempt at a solution

    I determined the reactions for R1 is zero and R2 is 2000. would this be correct?
     
  2. jcsd
  3. Feb 10, 2013 #2

    Simon Bridge

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    The reactions are usually what stops the beam from accelerating due to the listed applied forces.
    Depends on how you did it - your earlier question implies you don't know how to find the "reactions".
     
  4. Feb 11, 2013 #3
    Well I attempted to find a solution and wanted to know if those reactions where correct
     
  5. Feb 11, 2013 #4

    Simon Bridge

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    The answers are not so much "incorrect" as meaningless.
    No units, no working, no directions (force is a vector).

    You are being trained to able able to solve problems that nobody knows the answers to - so there is nobody to ask "have I got this right?"
    This means you need to figure out how to tell whether you have got it right or not.
    One way to do this is to see if your answers make sense for the situation described.

    If I assume your units are supposed to be "Newtons", then your answer means the beam has a 2000N force at the very end and a 2000N force (possibly in the opposite direction) 5m further in (at R2).
    What is the resulting motion of the beam under those forces?
     
    Last edited: Feb 11, 2013
  6. Feb 11, 2013 #5

    SteamKing

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    Take a deep breath.

    The OP clearly has units of feet and pounds in his problem statement. Presumably, the reactions are also going to be in pounds as well.

    What is unusual about the description of the beam and its loading is that the only concentrated load is applied directly over a support. Hence, the other support will provide no reaction (assuming a weightless beam) and all of the reaction is occurring under the applied load. This makes for rather simple shear and bending moment curves.
     
  7. Feb 11, 2013 #6
    The resulting motion of the beam under the forces would be zero. I believe my answers make since. I did omit, you are correct, there is a 2000 lb force being applied to an end of a beam. I do know that any reactions have to equal this force. I believe the reaction at the opposite end is 0 lbs and the reaction 5 '-0" away would be 2000 lbs in the opposite direction of the force being applied.
     
  8. Feb 11, 2013 #7
    to reply to SteamKing: the concentrated load of 2000 lbs is being applied to the end of the beam. There is a reaction 5'-0" way, which I determined to be 2000 lbs. The other reaction at the opposite end of the beam will provide no reaction hence the zero. The bending moment curve I determined would be from the 2000 reaction to -10000 lbs.
     
  9. Feb 11, 2013 #8

    Simon Bridge

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    Yes :)
    presumably...
    hmmm? <checks>
    ... I read that like this:

    if R1 is at x=0, then R2 is at x=10', and the load is 2000lbs (downwards) at x=15' (at the very end of the beam, 5' from R2)
    Wouldn't the load tend to create a moment about R2?

    The nature of the support is not described.
    Perhaps the beam is G-clamped to two sawhorses (fixed to the ground)?
    This would allow reacting forces in any direction.

    Of course, it could be I'm using a different definition of "reaction".
     
  10. Feb 11, 2013 #9
    Simon: you interpreted my discription correctly.
     
  11. Feb 11, 2013 #10

    Simon Bridge

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    i.e. the only concentrated load is not applied directly over a support?
    Wouldn't the load tend to create a moment about R2?
     
  12. Feb 11, 2013 #11
    correct. the only concentrated load is at the end on the beam, 5' from R2
     
  13. Feb 11, 2013 #12

    Simon Bridge

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    Please be clear about which questions you are answering.
    Wouldn't the load tend to create a moment about R2?

    With no reaction forces, the moment would be unbalanced wouldn't it?
    Does a 2000lb reaction (upwards) at R2 balance the moment all by itself?
     
  14. Feb 12, 2013 #13
    Inemecek: This thread is going off a bit, and you are partly responsible for not giving us a diagram. To return to your question - if the supports are simple supports there is no need for your question, because the reactions, if you have them, are checkable by you by applying the laws of equilibrium. That is: sum of forces in any two independent directions must be zero, and sum of moments about ANY point must be zero. If you have already used the sum of moments equation to obtain a reaction, then summing moments about any other point will reveal whether your reactions are correct.
     
  15. Feb 12, 2013 #14

    Simon Bridge

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    @inemacek: I agree with pongo38 - the process suggested in post #13 is what my questions are supposed to be leading you through.
     
  16. Feb 12, 2013 #15
    Please look at this problem as if it were a see-saw with mama weighing 2000 lb and child at the other end weighing....?. The algebraic sum of the moments about any point must be zero. Currently, your solution does not satisfy that.
     
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