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Supporting a plank of wood with a mass at the centre

  1. Mar 29, 2015 #1
    1. The problem statement, all variables and given/known data
    "Two people are holding a plank of wood at either end, with a mass located at the halfway point between them, they both walk backwards extending their arms, the mass still being halfway between them. The plank becomes harder to support, why is this?"

    2. Relevant equations
    Maybe the idea of moments?

    3. The attempt at a solution
    If it involves the use of moments do I deem the people are acting as pivots? As I haven't really given a "solution" could you perhaps point me in the right direction, so I can come to the answer?

    Thanks.
     
    Last edited: Mar 29, 2015
  2. jcsd
  3. Mar 29, 2015 #2

    mfb

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    Did you draw a sketch with the relevant forces?

    I see a possible real-life effect that could make it harder, but that is beyond the typical "find the forces" problems.
     
  4. Mar 29, 2015 #3

    Doc Al

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    I assume that at first the two people are supporting the plank at some point not at the ends of the plank? In any case, it is not obvious to me that it will become harder to support the plank as they move towards the ends of the plank. Is that all the information you have? Or are they holding the ends of the plank at all times, and just walking back with their hands now extended?
     
  5. Mar 29, 2015 #4
    I think the teacher was just trying to use an analogy to make it easier to understand. I am just in the process of creating images, which I will upload in a second.
     
  6. Mar 29, 2015 #5
    7i8ExlR.png

    It could well be possible I misunderstood the question the teacher asked, as we did have to write it down, if so I will ask next time I see him.
     
  7. Mar 29, 2015 #6

    mfb

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    Hmm, that would make sense. I don't see how the problem statement would lead to this interpretation, but then it is really getting harder.

    As shown in post 5, the forces stay exactly the same with an ideal plank. A real plank would bend more in the second setup, which could it make harder to carry.
     
  8. Mar 29, 2015 #7
    Maybe it is that then, as I said I probably misheard what the teacher said when I wrote the problem down.
     
  9. Mar 29, 2015 #8
    Sorry I have just looked over my notes and realised that I wrote: "two people are holding the plank at either end they move further apart", so it must mean they are just walking back". I'll edit it on the first post now
     
  10. Mar 29, 2015 #9

    Doc Al

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    Good. Now the problem makes some sense.
     
  11. Mar 29, 2015 #10
    Could you give me a hint of how to approach the question please?
     
  12. Mar 29, 2015 #11
    No need for help anymore I have figured it out! Thanks anyway.
     
  13. Mar 29, 2015 #12

    Doc Al

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    Good. I think you gave yourself your own hint: Moments.
     
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