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shubhamsrg

a block of mass 1kg is placed on a plank of mass 2kg, length of plank = 2m. Coefficient of friction between plank and block is 0.5 and the round on which the plank is kept is smooth. A constant force F=30N is applied on the plank; the time after which the block separates from the plank =?

  • one year ago
  • one year ago

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  1. shubhamsrg
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    |dw:1356509438621:dw|

    • one year ago
  2. A.Avinash_Goutham
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    concepts of physics?

    • one year ago
  3. shubhamsrg
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    ??

    • one year ago
  4. A.Avinash_Goutham
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    nthin i thot it was from hc verma well i think i missed smthin fk=5N? why wud they seperate?

    • one year ago
  5. shubhamsrg
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    pseudo force i'd say..

    • one year ago
  6. shubhamsrg
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    and i dont know from which book this ques came from,,it was asked in a test..

    • one year ago
  7. A.Avinash_Goutham
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    yup and wat's the acceleration? 10*1

    • one year ago
  8. shubhamsrg
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    how ?

    • one year ago
  9. shubhamsrg
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    shouldnt it be 30 -fk = 30 - 5 ?

    • one year ago
  10. shubhamsrg
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    =25

    • one year ago
  11. A.Avinash_Goutham
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    i dont think fk counts initially they both move as a single body

    • one year ago
  12. shubhamsrg
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    i see,,i didnt consider that.. anyways, how does it matter? the question will begin only when the block starts moving..

    • one year ago
  13. shubhamsrg
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    i mean the solution will begin* also, fk isnt high enough relative to the force, so shouldnt it start moving at t=0 only ?

    • one year ago
  14. A.Avinash_Goutham
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    opps sorry u r right nd u almost got it

    • one year ago
  15. shubhamsrg
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    well i'll show you my attempt..just that i didnt get the right ans..

    • one year ago
  16. A.Avinash_Goutham
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    find the independant acc's of both? and find the replative acc and use that goddamn formula

    • one year ago
  17. shubhamsrg
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    wait, can you confirm to me if the normal reaction acting on the block is 1*10N or (1+2)*10 N ?

    • one year ago
  18. A.Avinash_Goutham
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    wat's block 1? anyways i think i understand ur confusion well i guess it is 10 but u jus need to find fk....and draw fbd with the horizontal forces

    • one year ago
  19. shubhamsrg
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    allow me to try accn of block w.r.t to ground = 30 -5 /1 = 25 accn of plank w.r.t to ground = 30 - 5 /2 = 12.5 relative accn = 37.5 is that right?

    • one year ago
  20. A.Avinash_Goutham
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    idk...gime sm time i feel giddy

    • one year ago
  21. shubhamsrg
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    i still find something fishy in my solution.. hmm :|

    • one year ago
  22. A.Avinash_Goutham
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    • one year ago
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  23. shubhamsrg
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    how do you say force on block 1 is 10 N? shouldnt the pseudo force be 30 N ?

    • one year ago
  24. A.Avinash_Goutham
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    as far as i kno pseudo is ma

    • one year ago
  25. shubhamsrg
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    well,,i'd argue there,,i maybe wrong,, anyways, that still doesnt yield the right ans, so got to be some flaw there..

    • one year ago
  26. A.Avinash_Goutham
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    ohh u from fiitjee?

    • one year ago
  27. shubhamsrg
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    no buddy,, :|

    • one year ago
  28. shubhamsrg
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    am dropping my year for jee and ISI and CMI etc, living here in a remote place, just a small tuition like coaching here, no fitjee,resonance,akash here ,nearby i mean..

    • one year ago
  29. A.Avinash_Goutham
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    ohh i get it....it's okay......u have a better chance than any1 in here nd ur question....sry it's been long since i worked on these.........so i donno if i can help

    • one year ago
  30. shubhamsrg
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    hmmm ..nevermind,,thanks for trying sir! :)

    • one year ago
  31. sauravshakya
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    |dw:1356521627848:dw|

    • one year ago
  32. sauravshakya
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    So, accleration of the block with respect to the plank = -25/1 = -25ms^2

    • one year ago
  33. sauravshakya
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    Now, s=ut+1/2 at^2 2=0+1/2 25 t^2 t^2=100 t=10seconds

    • one year ago
  34. shubhamsrg
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    @sauravshakya you missed out on the fact that the plank also moves, so the block wont have to cover 2m completely, if you get what i mean.

    • one year ago
  35. Diwakar
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    When the block starts to move the maximum friction acts on it i.e.5N Then we draw the free body diagram as shown in the file attached. Note that we work in the frame of the plank because it is easier. Pseudo force is given by "ma". Where m is the mass of the body in concern(i.e. in this case 1kg) and "a" is the acceleration of the frame(i.e. the plank). We get the free body diagrams for plank and block as shown. Eqn of motion for plank is 30-5=2a giving a=12.5m/s/s Eqn of motion for block is 1*a-5=1*A where A is the acc of the 1kg block Inserting the value of "a" gives A=7.5m/s/s Length of plank is 2m Using 2nd eqn of motion we get time to cover 2m =0.73s(approx) ***DO TELL THE CORRECT ANSWER PLEASE**

    • one year ago
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  36. shubhamsrg
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    thats where i went wrong, so it was a conceptual problem from my side..thanks a lot @Diwakar yep, thats the correct ans

    • one year ago
  37. shubhamsrg
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    just confirm me this, taking into account the pseudo force means A is the relative accn of block right ?

    • one year ago
  38. shubhamsrg
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    relative to the plank..

    • one year ago
  39. Diwakar
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    Yes, because we are working in the frame of the plank.

    • one year ago
  40. shubhamsrg
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    i understand this concept a lot better now..thanks..

    • one year ago
  41. Diwakar
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    acceleration w.r.t. ground can be found out as a-A=12.5-7.5=5

    • one year ago
  42. Diwakar
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    .where a and A are the magnitudes. we used - sign because they point in opposite directions.

    • one year ago
  43. shubhamsrg
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    yep..again,,thanks a ton! :)

    • one year ago
  44. Diwakar
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    you are welcome!!

    • one year ago
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