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Homework Help: Free-body diagram for two boxes connected on a pulley.

  1. Oct 2, 2006 #1
    The plane is frictionless.

    Almost all of the choices seems right.
    It's just confusing. How can I know which one is correct?

    Thank you.

    Attached Files:

  2. jcsd
  3. Oct 2, 2006 #2

    Doc Al

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    Staff: Mentor

    A free body diagram should only show the actual forces acting on the body. Is "ma" a force?
  4. Oct 2, 2006 #3
    ma equals F, so I'd say ma is a force. Am I wrong?
    What do you mean by saying "actual" forces?
    Could you please tell me what actual forces are?
    How can I know if a force is an actual force or not?

    Thank you.
  5. Oct 2, 2006 #4

    Well the way that i'd work on this problem is by identifyiing every force acting upon each block

    Upon Block Block two, i'd say that there are 3 forces acting upon the block (3/4 of the possibilities agree with me, a,b and c)
    These forces are :

    Gravity - one would hope that we are on the earth... and that the earth pulls down...
    Force normal - else the block is so havy that it just goes crashing through the table
    The pull of the other block via tension on the string- it pulls the other way.
    I believe answer D is incorrect due to the fact that the string is already pulling on it w/ T, and that there is no other means for block one to affect block two,

    So have to two forces pull in the same direction that block one pulls, can't happen as there is really only one object pulling rightwards/downwards/clockwise

    Now, regarding the other block, block one, i'd say that there are just two forces acting on it, one upwards, and one downwards. (2 of the answers agree with me,a and b, the other two contradicting each other)

    Gravity, and the string, again

    Now i believe that tensionis a better term to use versus accelaration*mass, because, though M1a is a potentially valid label, it isn't conclusive, as you would still need to find the tension to solve for "a". On the other hand, tension can be found by determining the blocks' pull on the string, i.e. m1g, m2gx
    (two of the responses agree with me,b and d, the other two contradicting each other)

    the common answer throughout that i concur with is b.
    Last edited: Oct 2, 2006
  6. Oct 3, 2006 #5
    But I still don't get one thing: what forces to include in free-body diagram in presence of forces displaying Newton's 3rd law.
    A system constituting Newton's 1st or 2nd law is not difficult to draw free-body diagram, but how do you know what forces to include when forces constituting Newton's 3rd law also exist in the system?
    Last edited: Oct 3, 2006
  7. Oct 3, 2006 #6

    Doc Al

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    Staff: Mentor

    By "actual" forces, I mean forces that have an identifiable agent--where something is actually pulling or pushing. The tension from the rope is exerted by the rope, gravity is exerted by the earth, the normal force is exerted by the surface. The only "long range" force in the mix is that of gravity; all the other forces are "contact" forces--something (like a rope or a surface) actually touches the object in order to produce a force on it.

    "ma" is not a force. The net force on an object equals "ma", due to Newton's 2nd law. But don't show "ma" on a free body diagram!
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