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Homework Help: Contact Force

  1. Feb 7, 2008 #1
    [SOLVED] Contact Force

    1. The problem statement, all variables and given/known data
    a.Two blocks are in contact on a frictionless table. A horizontal force F Is applied to M2. The force's magnitude is 4 N. [The diagram shows the force pushing on the left side of M2, which has M1 on its right side.]If M1=1.09 kg, M2=3.02 kg, find the magnitude of the contact force between the two blocks.
    b. If instead an equal but oppositely directed force -F is applied to M1 rather than M2, find the magnitude of the contact force between the two blocks.

    2. Relevant equations
    Alright, right now we're doing Newton's laws of motion, so I'm pretty sure that F=ma is the only relevent equation.

    3. The attempt at a solution
    We barely brushed upon contact forces and they are not mentioned in our book that I have seen. I checked wikipedia (I know...) and it informed me that when there are two masses and the force is being applied to mass A, then the contact force will be proportional to Mass B. Thus, I went with the assumption that I could do this:

    A= 4/3.02 m/s^2

    I figured I could then apply this to the second block.

    F=4.36/4.02 N

    I inputted this and it was incorrect. So, I figured that it perhaps was the difference between the two forces. This answer (2.9154) was incorrect as well. I have not even tried part b yet, knowing that the method involved is the same. I went to study group on Tuesday and the group leader claimed that it would just be 4 N. As you can probably guess, that is incorrect as well. Any help you can give would be very appreciated.
    Last edited: Feb 7, 2008
  2. jcsd
  3. Feb 7, 2008 #2


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    What is the value of F? You can't put a number on the value of the contact force if the value of F is not given. It would have to be determined as a function F, using free body diagrams.
  4. Feb 7, 2008 #3
    I'm sorry- the value of F is 4 N. I forgot to type this in. I will add it right away.
  5. Feb 7, 2008 #4


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    OK, F=4N. That's the applied force. You are now looking for the contact force between the 2 blocks. Although the Wikepedia solution (that you noted) is correct, it is not a good starting point for a beginner. You've got to start with the basics, Newton's 2nd law, F_net = Ma. I'd first look as the system of the 2 blocks as a whole, and apply Newton 2 to that system (the blocks must move together). What do you get for the acceleration? Then since each mass alone must accelerate at that same value, draw a free body diagram of the 2nd block (that is , isolate it from the other block), and note the unknown contact force acting on it, and use Newton 2 again to solve for it. Do the same for Part B.
  6. Feb 7, 2008 #5
    Thank you very much.
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