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B Wooden Plank Problem

  1. May 7, 2017 #1
    I have a question about these plank problems. These questions are mechanical aptitude practice questions, I wanted to know understand the principle or methodology to solve these questions.

    I know the answers to these questions which are correspondingly C and B which were listed at the bottom of the question's page.

    It looks simple at a glance, but I can't find any explanation from google so I decided to post here.

    plank problems.png
  2. jcsd
  3. May 7, 2017 #2


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    Interesting. I don't even agree with the book answer for the first scenario, and the second scenario is questionable also.
    You could try to build these contraptions with some mechanics toys or indeed use your mechanical aptitude. I'm not surprised googling doesn't help :smile:

    I interpret 'fixed pivot' as equivalent to a pin driven into the underground: the point can't change location but the planks can rotate around the pivot point.
    And a 'moveable pivot' to me is like a rivet: a point that can change location and the planks can rotate around the pivot point.

    So for me the green triangle
    upload_2017-5-7_19-15-2.png is immobile and I come to answer e !

    The reason I find the second scenario questionable is that 'you' ( the 'she' in answer e ? ) have to be small enough to 'climb' on the righthand side, but big enough to get to the cube without putting your weight on the horizontal plank !
  4. May 7, 2017 #3


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    Hi and welcome to PF
    Is there something else we should be told about this problem?
    In the first picture, you effectively have a triangle on the left, consisting of two fixed points and three fixed side lengths. How could it move?
    In the second picture, the right side of the top plank is fixed so whatever you do when climbing up the right side, it won't drop and disturb the box. But what is keeping the left side of the top plank upright from the start?
  5. May 7, 2017 #4
    Thanks for the reply, I get your logic for for question 1. Just now I checked again because of the seed of doubt given by your reply and the listed answers C and B is what is written at the bottom.

    I'm not sure if it would help but, I do have only have 1 example, but even then I don't understand the writers logic with the poor explanation given.

    Attached Files:

  6. May 7, 2017 #5
    @sophiecentaur This is meant to be a mechanical aptitude test based questions, strictly on only the principles, therefore I don't have additional information.

    I did pay $30 to get these type of questions to practice. If you're wondering, I applied for the army reserve and am practicing a bit for the additional aptitude test for combat engineer.
  7. May 7, 2017 #6
    I read your question further just now and I find it puzzling as well. I probably didn't pick a very good website and don't really want to spend more money. I did not get much information besides what has been posted. Only more questions like this from the online quiz Question 3.png Question 4.png
  8. May 7, 2017 #7
    Not sure if this helps but, this is also another question similar in nature to the 2nd question I initially posted from the quiz. I hope these questions can be logically solved :nb)
    Question 5.png
  9. May 7, 2017 #8


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    For what it's worth (certainly not 30 $ !): I don't understand the answer in #4 and I agree with the answers in #6 and the answer in #7
  10. May 7, 2017 #9
    Thanks for the feedback. I kind of get what the questions are asking, you'd have to think about what will happen to the other pivots momentum and their position before and try to picture what will most likely happen after when the corresponding force is applied.

    If the end pivots are fixed, then the whole object must rotate and the pivots should fall either clockwise or counterclockwise.
  11. May 8, 2017 #10
    the green triangle can change shape if plank B is pushed up and A rotates around the bottom star.
    i think they're trying to show little notches on the planks that you can slide through the stars. But they should say this in the question.
  12. May 8, 2017 #11
    I find the second puzzle in the OP utterly bewildering. If this setup is vertical, it's not stable to begin with since the left part will immediately start to collapse. Is the implicit assumption here that the friction would hold it up just barely, but the moment you climb on it will no longer be stable?
  13. May 8, 2017 #12


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    Once I have read a question as badly put as that, I don't really like carrying on as I have lost confidence. It's a shame because, with a bit better instruction, (nothing should be 'implicit') those questions would be worth trying to answer.
  14. May 8, 2017 #13
    Same here, @sophiecentaur . This reminds me of my own military aptitude test way back when (this was for mandatory service), where you were supposed to listen to tiny pitch changes to tell whether a ship was approaching or receding. Of course, the equipment was ancient and the sounds were played from a wobbly tape. So, if you were actually good at this, you were faced with the dilemma of "do I only mark the big changes, or be really precise?", with the latter actually reducing your score. Totally made me lose interest.
  15. May 8, 2017 #14


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    Bad questions get bad answers. I have the same reaction against almost all role playing and simulation exercises because they never tell you the rules fully enough.
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