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Calculate the acceleration of the block

  1. Mar 21, 2007 #1
    A 0.5 kg block is being slid up a chalkboard with an applied force of 6.0 N upward and 2.0 N inward towards the board. If the coefficient of kinetic friction is 0.4.

    a) Calculate the force of friction.

    b) Calculate the acceleration of the block.


    F = mju R [ F = u R]

    1. F/u = 6/0.4 = 15N ???

    2. F = ma ..... 6/0.5 = a = 12m/s^2

    not sure... some1 back me up on this!
     
    Last edited: Mar 21, 2007
  2. jcsd
  3. Mar 21, 2007 #2
    no one helping
     
  4. Mar 21, 2007 #3

    Doc Al

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    Staff: Mentor

    If you would like help, show what you've done so far--per PF rules.

    Start by identifying all the forces acting on the block.
     
  5. Mar 21, 2007 #4
    F = mju R [ F = u R]

    1. F/u = 6/0.4 = 15N ???

    2. F = ma ..... 6/0.5 = a = 12m/s^2

    not sure... some1 back me up on this!
     
  6. Mar 21, 2007 #5
    Show us your attempts at solving this problem first.
     
  7. Mar 21, 2007 #6
    2. (6-(2*0.4))/0.5= 10.4 upwards w.r.t board.

    (6-(2*0.4)-(0.5*9.8))/0.5=0.6 upwards w.r.t ground.
     
  8. Mar 21, 2007 #7

    Doc Al

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    I assume F is the friction force? So what is R?
     
  9. Mar 21, 2007 #8
    sorry
    mean Fn
     
  10. Mar 21, 2007 #9
    can u help now
     
  11. Mar 21, 2007 #10

    Doc Al

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    Staff: Mentor

    Seems like you have several versions of solutions. Approach it systematically: What forces act on the block? What's the force of friction?
     
  12. Mar 21, 2007 #11
    can u help me
    which formula i need to use first one
     
  13. Mar 21, 2007 #12

    hage567

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    Homework Helper

    For (1), use the formula you've already stated, F=uN. Do you know what force N is in this case? (I think you were calling in R in your first post)
     
  14. Mar 22, 2007 #13
    I AM NOT SURE
    CAN U DO THAT.
     
  15. Mar 22, 2007 #14

    hage567

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    N is the normal force. The block is being pushed against a vertical wall. So which of the two forces you were given in the question is holding the block on the wall? That is the force you need to use to find the force of friction.
     
  16. Mar 22, 2007 #15
    i need help in B
     
  17. Mar 22, 2007 #16

    Doc Al

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    Staff: Mentor

    So you have finished A and correctly calculated the force of friction? If so, good.

    To solve B, identify all the forces acting on the block. Then find the net force and use that to calculate the acceleration via Newton's 2nd law.
     
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