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Homework Help: Forces concept question

  1. Sep 23, 2006 #1
    The problem is to rank the forces exerted from three blocks on top of each other.
    Like so:

    [10 kg] #3
    [5 kg] #2
    [2kg] #1

    Here is my reasoning,
    From least force, to most force:
    a. Force of block #1 on #3; They don't touch, so net force = 0
    b. Force of #3 on #1; same thing, net force = 0
    c. Force of #1 on the floor; which is just m1*g
    d. Force of floor on #1; normal force against just block #1? Equal to c?
    e. Force of #2 on #1; m2*g
    f. Force of #1 on #2; normal force against #2?
    g. Force of #3 on #2; m3*g
    h. Force of #2 on #3; normal force against #3?

    Is that how it is suppose to work? It doesn't seem right, but I can't figure out which concept I am understanding wrong.
  2. jcsd
  3. Sep 23, 2006 #2

    Andrew Mason

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    Do a freebody diagram for block 1 (a diagram showing all the forces (vectors) acting on block #1). What is the net force? (hint: does it accelerate? - I think we are to assume that the blocks are sitting on a surface at rest). What does this tell you about the magnitude of the downward and upward forces? What is the upward force? What are the downward forces on #1?

  4. Sep 24, 2006 #3
    Ok, there is no acceleration, because nothing is moving, so the net force of the entire stack must be 0. For block #1, the weights of #2 & #3 combined is the force downward on it, and the force of the floor on #1 is the normal force of the weight of #'s 1-3? Does each block not have any normal force against the block on top of it then?
  5. Sep 24, 2006 #4

    Andrew Mason

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    You just have to do a free body diagram for each block and set the resulting force to 0. The downward forces on each block are its weight plus the weight of those blocks above it. The upward force is the normal force from the floor. They sum to 0. To figure out what the normal force is on a block, do the free body diagram for that block.

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