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Can the normal force between a blocks accelerating together be zero?

  1. May 15, 2008 #1
    1. The problem statement, all variables and given/known data
    Three rectangular blocks of equal mass m slide to the right along a frictionless horizontal surface accelerated by a force F applied to the left side of the left-most block. (a) Draw a free-body diagram for each block. (b) Determine the magnitudes and directions of all of the forces on each block in terms of m, F and/or g.


    2. Relevant equations



    3. The attempt at a solution
    As for vertical forces, each block has the force of gravity downwards and the normal force from the surface pointing upwards, which sum to 0. That part is fine.

    As for the horizontal forces, on block 1 I have the applied force to the left, and the normal for from block 2 to the right. For block 2 I have the normal force from block 1 to the right and the normal force from block 3 to the left. And then for block 3 there I have the normal force from block 2 to the right. Using Newton's Law of equal and opp. pairs, I know that the normal force of 2 on 1=-(force on 1 on 2) and normal forces of 2 on 3=-(force 3 on 2).

    By equating these forces and using Newton's Law, I get that the force of 2 on 3 is just F (applied force) and 3 on 2 is -F. And 2 on 1/1 on 2 is zero. Is that possible? Am i missing something?
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. May 15, 2008 #2
    you made a mistake somewhere doing this.

    The easiest way to do it is to start by calculating the acceleration of all the blocks by using F = m*a for all of them together
     
  4. May 16, 2008 #3

    tiny-tim

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    Science Advisor
    Homework Helper

    Hi mistymoon_38! :smile:

    Hint: Call the acceleration A.

    So the net force on the three blocks as a whole is F = 3mA.

    Then what is the net force on the first block, to produce an acceleration of A? :smile:
     
  5. May 19, 2009 #4
    Hey! We have the same problem for my Physics Recitation.

    I disagree with your block 1 which might lead to your error. I said that The Force Applied is going to the right and The Force (2 on 1) is going to the left.

    This means that F(1 on 2) = -F(2 on 1) so it should not equal zero but be the same.
     
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