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Homework Help: Determine the magnitude of the contact force between the two blocks.

  1. Oct 1, 2012 #1
    1. The problem statement, all variables and given/known data
    Two blocks of masses m1 and m2, with m1 > m2, are placed in contact with each other on a frictionless, horizontal surface. A constant horizontal force is applies to m1.

    a) find the magnitude of the acceleration

    b) determine the magnitude of the contact force between the two blocks.

    2. Relevant equations

    ƩF =ma

    3. The attempt at a solution

    Well, for a), because you are simply finding the magnitude of the acceleration of both particles, you can combine both masses and apply the above formula.

    ƩF =(m1+m2)a


    Regarding b), I understand these particles need to be modeled individually, but how exactly would I do so.

    My main issue is understanding the force diagrams drawn. Let's just say for example... M1.

    M1 has mg acting upon it, and N pushing up. There is a force pushing the object to the right, but there is also a force labeled as P21. Could someone clear some of this up for me?

    I don't even know anymore, so frustrated by the lack of teaching skill with this professor. Perhaps somebody could throw in some tips on understanding physics? -_-
  2. jcsd
  3. Oct 1, 2012 #2
    ƩF =ma

    An object will accelerate when a net force is exerted on it.
    You have found the acceleration, you can find the force acting on it.

    The contact force is the same on both sides as in Newton'n 3rd Law.
  4. Oct 1, 2012 #3


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    The use of Free Body Diagrams (FBD) is essential to the understanding and solution of forces acting on objects or particles. In a FBD, part of the system is isolated or separated from the rest of it, and then all forces acting on it, known and unknown, are identified and shown on the diagram, with their directions indicated where known. When the system is separated this way, forces that were internal to the system as a whole, become external forces on the FBD, and are shown in addition to any other external forces acting on the isolated part. When there is contact between objects, there is contact force acting on the object. That's what P_21 represents...the perpendicular or normal contact force of block 2 on block 1. Solve for it using Newton 2 applied to your FBD object.
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