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Homework Help: Friction Concepts

  1. May 21, 2014 #1
    1. The problem statement, all variables and given/known data
    A mass M is initially at rest on a horizontal surface, μs=0.30 and μk=0.20 . A horizontal string then pulls M with a tension T. N is the normal component of the contact force. All quantities below are magnitudes.
    True or False:
    1) If M does not accelerate, then T ≤ μsN
    2) M will accelerate if T exceeds μkN
    3) M will accelerate if T exceed μsN
    4) The value of N equals Mg
    5) T equals μkN if M remains at rest.
    6) The total force on M is μkN if M remains at rest.

    2. The attempt at a solution
    From what i have learned i thought the answer should be:
    1) True (to gain acceleration, first F must exceed μsN)
    2) True (it takes µkN, force to just move M, and anything over that will accelerate M)
    3) True
    4) True (mg = N, the normal force)
    5) False (T is less than or equal to µsN for M not to move)
    6) False (The total force on M is equal to or less than µsN for no movement)
    The in incorrect but i am not sure as to why they are wrong.
  2. jcsd
  3. May 21, 2014 #2
    looks good
  4. May 21, 2014 #3
    The correct answers are actually
    1) True
    2) False (It must exceed ukN not just uks)
    3) True
    4) True
    5) False
    6) False
  5. May 21, 2014 #4


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    Homework Helper

    The actual reason (which you may have meant to say) is it must exceed u[itex]_{s}[/itex]N, not u[itex]_{k}[/itex]N
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