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Homework Help: Newton's laws and working with systems

  1. Mar 16, 2008 #1
    1. The problem statement, all variables and given/known data

    Say you have three carts connected by strings, with each cart's mass being 50kg, which way would the tension for be pointed in a free body diagram? By the way, the carts are accelerating..


    accelerating --> @ 3m/s/s
  2. jcsd
  3. Mar 16, 2008 #2


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    In this problem there is more then one object with forces acting on it. Which box are you drawing a free body diagram for?
  4. Mar 16, 2008 #3
    I drew one for the entire system
  5. Mar 16, 2008 #4


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    OK, well the tension is not going to point in the same direction for each block. For example:

    Which way does the tension from rope one pull block one? Which way does the same tension force pull block 2?

    You need to add more than one tension vector to the diagram, one for each block the tension force pulls on.

    I suggest making a separate free body diagram for each block, since it would not be possible to draw the tension in the free body diagram for the system as whole because they are internal forces.
  6. Mar 16, 2008 #5
    I'm not quite sure I understand how to do this.
  7. Mar 16, 2008 #6
    so when you are solving for the tension in the 2 strings, would one of the equations have m=the systems entire mass, and the other will have m=only 2 blocks?
  8. Mar 16, 2008 #7


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    Could you please post the entire problem? I can't offer much help if I don't know what you are trying to solve for.

    I am also going to need to see some of your work to give further help. What have you done so far? What forces are involved, etc.
    Last edited: Mar 16, 2008
  9. Mar 16, 2008 #8
    4. Given the following system of 50kg carts that accelerate at 3m/s/s with a coefficient of friction of .1 then;



    a) find the force that creates the acceleration on the system. (as if it were frictionless)

    b)calculate the total frictional force and the force of f1.

    c)calculate the tension in string A and B.
  10. Mar 17, 2008 #9


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    For part c) you need to think of each block separately.

    HINT: Find the sum of the forces on each block separately. Then use Newton's second law to relate those sums to the acceleration. You should now have a system of three equations. Can you use that to find your tension values?
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