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Two block on each other with friction

  1. Jul 12, 2015 #1
    1. The problem statement, all variables and given/known data

    2. Relevant equations
    F = ma
    Ffr = u * Fn

    3. The attempt at a solution
    After drawing FBD
    For m2: we have only friction force in horizontal axis so Ffr = ma ==> u * Fn = ma ==> u * mg = ma ==> u * g = a ==> a = 0.5 * 10 = 5 m/s2
    Now, moving to m1:
    F - Ffr = ma ==> F - u Fn = 10 * 5 ==> F - 0.5*130 = 50 ==> F - 65 = 50 ==> F = 115 N

    But it is not in choices,

    Is my solution wrong?
    Are the choices wrong?
  2. jcsd
  3. Jul 12, 2015 #2
    One of the choices is right. Your solution is wrong.

    a = Fnet/m

    What mass is needed in this equation?
  4. Jul 12, 2015 #3


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    Gold Member

    Did you use the correct value for the normal force here?
  5. Jul 12, 2015 #4
    I though that the mistake is with the Fn.
    Fn here is the sum of two weights as net vertical force is zero

    Dr Courtney, you considered them as single mass I see. The answer will be 65 N then. but I want to know my mistake
  6. Jul 12, 2015 #5


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    Gold Member

    Think about whether the friction force acting on m1 is coming from m2 or coming from the ground.

    What is the normal force acting between m1 and m2?
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