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Newton Laws in an elevator

  1. Oct 22, 2008 #1
    Hello guys, I am currently busy with this introductory assignment on Newtons Laws.
    I have tried to solve but i am not sure whether i have the right solution.
    Please help.

    1. The problem statement, all variables and given/known data

    Three blocks are stacked on top of each other inside an elevator as shown in the figure.
    Answer the following questions with reference to the eight forces defined as follows.

    the force of the 3kg block on the 2kg block, F of 3 on 2,
    the force of the 2kg block on the 3kg block, F of 2 on 3,
    the force of the 3kg block on the 1kg block, F of 3 on 1,
    the force of the 1kg block on the 3kg block, F of 1 on 3,
    the force of the 2kg block on the 1kg block, F of 2 on 1,
    the force of the 1kg block on the 2kg block, F of 1 on 2,
    the force of the 1kg block on the floor, F of 1 on floor, and
    the force of the floor on the 1kg block, F of floor on 1.

    Assume the elevator is at rest. Rank the magnitude of the forces.
    Rank from largest to smallest.

    3kg block
    V
    2kg block
    V
    1kg block
    v
    elevator floor


    3. The attempt at a solution

    F of floor on 1 and F 1 on floor (equal), F of 3 on 2 and F of 2 on 3 (equal), F of 2 on 1 and F of 1 on 2 (equal), F of 3 on 1 and F of 1 on 3 (equal).

    Is this ranking right? Thanks
     
  2. jcsd
  3. Oct 23, 2008 #2

    alphysicist

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    Hi iluvphys,

    I don't believe these are quite right. The ones you have identified as equal are equal, but there is one grouping out of place. How did you decide on the relative strengths?
     
  4. Oct 23, 2008 #3
    Good morning alphysicist,
    a)well I started thinking that most of the weight is on the 1kg block therefore the force that it puts on the floor must be equal to what the floor puts back on the block (N´s 3rd law).
    b)However i wasnt really sure about the last one since I believed that the 3kg block doesnt directly put its force on the 1kg block,it is rather the force of block3 and the force of block 2 acting on the 1kg block.

    Could you please tell me where my mistake is? Thank you
     
  5. Oct 23, 2008 #4

    alphysicist

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    Those both sound right to me. It's the forces in the middle that I am thinking about.

    Think about what each block is supporting. For example, the floor is supporting all three blocks (6kg of mass), so the floors force on the 1 kg mass is the largest force (and equal to the 1kg mass's reaction force on the floor).

    Now think about what the 1kg mass is supporting and what the 2kg mass is supporting. Do you see what's wrong?
     
  6. Oct 23, 2008 #5
    Oh you´re right.
    I wasn´t really considering the two in the middle they seemed not really "important" in the beginning. But now i understand that Block 1 has a bigger impact on block 2 than block 3 has on 2, and the other way round!
    Well, thanks again.
     
  7. Oct 23, 2008 #6

    alphysicist

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    That's right, because the upward normal force from block 1 is supporting both blocks 2 and 3, while the upward normal force from block 2 is only supporting block 3.

    Glad to help!
     
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