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What limit must the force excede if we want to separate two bodies + friction

  1. Oct 15, 2009 #1
    1. The problem statement, all variables and given/known data

    On a leveled base is a plank with a mass M = 2 kg, on the plank is a weight with a mass m = 1 kg. What limit must the force exceed, when we pull the plank in a horizontal direction for the weight to slip from the plank? Ratio of friction between the plank and the base is k1 = 0.4 and ratio of friction between the weight and the plank is k2 = 0.3.


    2. Relevant equations

    N= mg
    Fk= μkN

    3. The attempt at a solution

    M= 2 kg
    m= 1 kg
    μk1= 0.4
    μk2= 0.3

    For the plank and weight to start moving: F1= (M+m)gμk1= 11.76 N
    For the weight to start moving on the plank: F2= mgμk2= 2.94 N

    So if my logic works correctly I simply add the two forces and the force will be big enough for the weigh to start falling off the plank.

    F= F1 + F2= 11.76N + 2.94 N= 14.7 N

    And the answer would be: The force have to be bigger than 14.7 N for the weight to fall off the plank.
     
  2. jcsd
  3. Oct 15, 2009 #2
    I think it's right.
     
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