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Please help me with this problem in static friction

  1. Oct 9, 2016 #1
    1. The problem statement, all variables and given/known data
    2 Blocks connected by a string is placed on a rough horizontal floor,the coefficient of friction for block 1 is 0.2 while for block 2,it is 0.1. A force of 8N is applied on block 1 and a force of 1N is applied on block 2.Find the tension in the string and the frictional forces.
    See figure.
    2. Relevant equations
    newton's law
    friction normal equation



    3. The attempt at a solution
    I have the solution and it says there will be limiting friction case in block 1 since the net force acts in it's direction and there is a tendency of motion towards 8N force ,but I don't understand this if there is a tendency of movement then won't both frictional force and tension act simultaneously,so how can we say friction force will be maximum.
     

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  2. jcsd
  3. Oct 9, 2016 #2
    if you think otherwise i.e. there seems to be tendency to move towards left side -what is the argument?

    draw free body diagram and try to work out the answer/or arguments -also remember that frictional forces are 'self adjusting' and acts on 'tendencies' of motion..
     
  4. Oct 9, 2016 #3

    haruspex

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    Quite so. The question is dumb, since there is no way to answer it without more information about the set-up.

    In the real world, all strings have a tiny bit of elasticity, and even static friction allows a minute bit of elastic movement before yielding. To calculate what will happen, you would need to know all parameters that describe that. Even the length of string in the middle becomes significant.
     
  5. Oct 10, 2016 #4
    Sir,I think the argument given in the solution is not the correct explanation.But answer may be correct.
     
  6. Oct 10, 2016 #5
    Sir,thank you for replying,if the string is assumed to be an ideal string with zero elasticity, is this explanation correct?
     
  7. Oct 10, 2016 #6

    haruspex

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    The trouble with ideal behaviours is that they can lead to indeterminacy, as here. Consider an ideal table, four legs, on a level floor. What is the compression force in each leg? No way to tell.
    The only way the real world figures out what will happen In such cases is because nothing is ideal.
     
  8. Oct 10, 2016 #7
    Yes but shouldn't we consider one if question demands it,this is a problem from a competitive examination and we always assume ideal cases.So if we assume ideal cases,will that explanation be correct?
     
  9. Oct 10, 2016 #8
    i think the 'argument' you are referring to is about the 'condition of limiting friction' invoked.....
    in your opinion what should be the frictional force on block 1 ? give Fbd also...
     
  10. Oct 10, 2016 #9

    haruspex

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    No. The question is a nonsense.
    In an examination, with no opportunity to say 'indeterminate', the best you can do is to guess what the examiner might be thinking (and complain about the question later).
     
  11. Oct 10, 2016 #10

    mfb

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    I agree with haruspex. There is a range of possible values of tension in the first example. You can calculate this range, but you cannot determine the tension.

    As an easier system to study this, consider the blocks without external forces. Clearly you can have tension in the string - but the string does not have to have tension.
     
  12. Oct 10, 2016 #11
    so this question does not have determinate answer right?That is what I was thinking,friction need not be limiting.
     
  13. Oct 10, 2016 #12
    Thank you very much,I thought the same.
     
  14. Oct 10, 2016 #13

    CWatters

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    If there is no tension in the string what stops the 8N force on the right hand block overcoming static friction (max 6N) and increasing the tension to 2N ?

    Edit: OK I think I misunderstood you. Your comment that the string does not have to have tension only applies when there are no external forces.
     
  15. Oct 10, 2016 #14

    CWatters

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    Anyone else agree that the range of possible tensions is 2 to 3N?
     
  16. Oct 10, 2016 #15

    mfb

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    I got the same result.
     
  17. Oct 10, 2016 #16

    Vanadium 50

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    I disagree with everyone. There is an answer with a specific tension, provided you use only the information provided in the problem. If you use other information, in particular the title of the section, "static friction", you will be confused.
     
  18. Oct 10, 2016 #17

    mfb

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    Like the information that the coefficients of friction are static friction, as implied by "Equilibrium under static friction" in the problem statement?

    There is a specific answer if you assume that the coefficients are for dynamic friction, but I don't see how this assumption is justified. This interpretation also disagrees with the given answer.
     
  19. Oct 10, 2016 #18

    Vanadium 50

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    I'm wrong - there is (barely) enough friction to stop the system, so it's static. That means there is a range of allowed tensions.
     
  20. Oct 10, 2016 #19

    haruspex

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    Yes.
    One way to think about it is to allow some very small amounts of flexibility: some shear modulus in each block and some elasticity in the string.
    If either the string or the smaller block is the most flexible by some ratio, the larger block will start to slip before the tension even reaches 2N. If we suppose the kinetic friction is the same, or wind back the flexibility enough to prevent a result where one block slips first, it gets to 2N.
    If the larger block is much the most flexible then the tension will reach 3N and the smaller block will slip first.
     
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