1. PF Contest - Win "Conquering the Physics GRE" book! Click Here to Enter
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook