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shubhamsrg

  • 2 years ago

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 =?

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  1. shubhamsrg
    • 2 years ago
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    |dw:1356509438621:dw|

  2. A.Avinash_Goutham
    • 2 years ago
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    concepts of physics?

  3. shubhamsrg
    • 2 years ago
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    ??

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

  5. shubhamsrg
    • 2 years ago
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    pseudo force i'd say..

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

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

  8. shubhamsrg
    • 2 years ago
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    how ?

  9. shubhamsrg
    • 2 years ago
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    shouldnt it be 30 -fk = 30 - 5 ?

  10. shubhamsrg
    • 2 years ago
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    =25

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

  12. shubhamsrg
    • 2 years ago
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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..

  13. shubhamsrg
    • 2 years ago
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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 ?

  14. A.Avinash_Goutham
    • 2 years ago
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    opps sorry u r right nd u almost got it

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

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

  17. shubhamsrg
    • 2 years ago
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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 ?

  18. A.Avinash_Goutham
    • 2 years ago
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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

  19. shubhamsrg
    • 2 years ago
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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?

  20. A.Avinash_Goutham
    • 2 years ago
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    idk...gime sm time i feel giddy

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

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

  24. A.Avinash_Goutham
    • 2 years ago
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    as far as i kno pseudo is ma

  25. shubhamsrg
    • 2 years ago
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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..

  26. A.Avinash_Goutham
    • 2 years ago
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    ohh u from fiitjee?

  27. shubhamsrg
    • 2 years ago
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    no buddy,, :|

  28. shubhamsrg
    • 2 years ago
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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..

  29. A.Avinash_Goutham
    • 2 years ago
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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

  30. shubhamsrg
    • 2 years ago
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    hmmm ..nevermind,,thanks for trying sir! :)

  31. sauravshakya
    • 2 years ago
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    |dw:1356521627848:dw|

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

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

  34. shubhamsrg
    • 2 years ago
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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.

  35. Diwakar
    • 2 years ago
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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**

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  36. shubhamsrg
    • 2 years ago
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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

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

  38. shubhamsrg
    • 2 years ago
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    relative to the plank..

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

  40. shubhamsrg
    • 2 years ago
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    i understand this concept a lot better now..thanks..

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

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

  43. shubhamsrg
    • 2 years ago
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    yep..again,,thanks a ton! :)

  44. Diwakar
    • 2 years ago
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    you are welcome!!

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