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Homework Help: Newton's Third Law Question

  1. Nov 19, 2008 #1
    1. The problem statement, all variables and given/known data
    Two blocks are accelerated across a horizontal frictionless surface as shown. The coefficient of static friction between the two blocks is 0.7, and the coefficient of kinetic friction is 0.5, use M= 1.0 kg. When F=1.2 N, frictional forces keep the two blocks from sliding relative to eachother, and the two move with the same acceleration. In this case, what is the acceleratiob of the two block system? (The picture is of mass 2m on the bottom with mass M on top of it, and F pointing to the right)
    2) What is the horizontal component of the force the large block exerts on the small block
    3) Suppose that F is increased. What is the maximum acceleration that mass 2M can have, without mass M slipping off?

    2. Relevant equations


    3. The attempt at a solution
    I got the first portion of the problem right. I just solved for f=ma, and plugged in the f=1.2. I figured that the coefficients of friction given were not relevant here because the system is accelerating. For an answer I got a=.4 m/s2 to the right.
    2) For the second part of the question: the horizontal component the large block exerts on the small block, I put .4 to the left. I thought that according to Newton's third law the force is equal in magnitude and opposite in direction, but it is saying this answer is wrong. It says the correct answer is .4 to the right. Why is it the same?
  2. jcsd
  3. Nov 19, 2008 #2

    Doc Al

    User Avatar

    Staff: Mentor

    I don't understand how you are applying Newton's 3rd law in this case.

    Realize that both blocks have the same acceleration, which equals 0.4 m/s^2 to the right. To find the force on the top block (which is exerted by the bottom block), apply Newton's 2nd law.
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