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Action reaction problem or Newton's 3 law

  1. Oct 26, 2015 #1
    1. The problem statement, all variables and given/known data

    There is a bloc B that is on a surface S. A bloc A is on on the bloc B. We pull the rope towards the right. There is friction between all surfaces and the blocs stay stationary (They don't move). Identify all the forces which are applied on the bloc B.
    http://imgur.com/Hzgc8pP

    2. Relevant equations

    No equations.
    3. The attempt at a solution

    I know that there is going to be a Normal upward, another normal that is downward because it is a reaction according to Newton's third law, the gravitational force downward. I think that there is going to be a friction force on the right because it is a reaction to the friction of bloc A. Now, I don't know what else to find.... There is one missing...

    Here is another image http://imgur.com/FYWfLgE
     
  2. jcsd
  3. Oct 26, 2015 #2
    Consider any pair of Newton's Third Law action/reaction forces. The two forces never act on the same object.

    So, for example, Block A pushes down on Block B, and Block B pushes up on Block A.
     
  4. Oct 26, 2015 #3
    Yes but I dont see what else I can get... I'm only interested in object B and I got one pair of normal forces on the bloc A and B.
     
  5. Oct 26, 2015 #4
    Draw a diagram showing only Block B. On the diagram indicate only the forces that are acting on Block B.
     
  6. Oct 26, 2015 #5
    I know that there is going to be a Normal upward, another normal that is downward because it is a reaction according to Newton's third law, the gravitational force downward. I think that there is going to be a friction force on the right because it is a reaction to the friction of bloc A.


    http://imgur.com/T9RXe3F
     
  7. Oct 26, 2015 #6
    Exerted by the table.

    Exerted by Block A

    (Every force is a reaction according to Newton's Third Law!)

    Exerted by Earth.

    Exerted by Block A.

    And one more ...

    Hint: The net force on Block B must be zero because Block B is not in motion.
     
  8. Oct 26, 2015 #7
    Oh, there's a friction force to the left because if there wasn't one, then the block B would be moving, right ? So it's not a reaction but more of a "stabilizer", right ?

    Also, when I said "I think that there is going to be a friction force on the right because it is a reaction to the friction of bloc A." and you said : "Exerted by Block A." You mean that the block A is responsible for this reaction to the right, right ?
     
  9. Oct 26, 2015 #8
    Table pushes to the left on Block B, Block B pushes to the right on Table. It's an action/reaction pair. We call it friction.

    Correct!

    Block A pushes to the right on Block B, Block B pushes to the left on Block A. It's an action/reaction pair. We call it friction.
     
  10. Oct 26, 2015 #9
    Just one thing, the "S" isn't a table but a floor.

    Forget it I understood what you meant Thanyou!
     
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