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PLEASE HELP! friction question

  1. Oct 31, 2010 #1
    1. The problem statement, all variables and given/known data

    A 2.5 kg block is pulled to the right by a horizontal string. The coefficient of static friction between block and floor is 0.30 and coefficient of kinetic friction is 0.20.

    What value of force applied by the string will start the block moving?

    2. Relevant equations

    f=ma
    Ff=(coefficient of friction)(Fn)

    3. The attempt at a solution

    i wasn't sure if i had to use coefficient of static or kinetic. i'm assuming kinetic since the block will be moving.
    so i got:
    Fn=(2.5)(9.8)
    Ff=2.5*9.8*.3
    =7.35
     
  2. jcsd
  3. Oct 31, 2010 #2

    PhanthomJay

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    You assumed the wrong type of friction but used the correct value of the coefficient. Up until the block actually moves, static friction applies.
     
  4. Oct 31, 2010 #3
    so i'm right? because i did use the value for the static friction.
     
  5. Nov 1, 2010 #4

    PhanthomJay

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    Yes, 7.35 N
     
  6. Nov 1, 2010 #5
    Because your moving the block you need to use kinetic if the block were to be sitting alone you would use static frction.

    in this problem, you were right you need to use the the coefficient of static friction until you actualy start moving the block, and then the static changes to kinetic, so you ar eright the answer is 7.35N
     
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