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Homework Help: Newtonian Forces on Blocks

  1. Oct 10, 2013 #1
    1. The problem statement, all variables and given/known data
    Block B (1kg) is held in contact with block A (3kg). When block B is let go, A is pushed to the right with a force of 160N. Static friction is .50 between the 2 blocks.
    Block A pushes on B such that B is not touching the surface upon which A is moving on. It's held in the air with only the normal force from A supporting it.
    Find the force of friction on Block B, the normal force exerted by A on B, and vertical force exerted by B on A.

    2. Relevant equations
    Newtonian forces of friction, gravity, etc

    3. The attempt at a solution
    For the Normal force of A on B:
    to find a: Fnet = 160N = m(A+B)a --> a = 160/4 = 40m/s^2
    mAa = F - F(BonA)
    FBonA = 160 - (3x40) = 40N = FAonB from Third Law

    For Force of Friction on B: Since friction would be working against gravity and there is no vertical motion, wouldn't the force of friction be mBg, its weight?
    The Max Ff would be Ff=μN = .50(40)=20N.
    Not sure about the vertical force of B on A.
  2. jcsd
  3. Oct 10, 2013 #2

    Simon Bridge

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    Science Advisor
    Homework Helper

    How can friction oppose gravity - isn;t this the normal force or did you leave out part of the description?
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