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Homework Help: Force, Mass problem

  1. Jun 19, 2011 #1
    1. The problem statement, all variables and given/known data

    The surface of an inclined plane shown below is frictionless. if F= 29N, what is the magnitude of the force exrted on the 3.0-kg by the 1.8kg block?

    2. Relevant equations
    I suppose F=MA

    3. The attempt at a solution
    I'm utterly confused here. The equation I have for force doesn't factor in angles, there may be a variation of the force equation that I do not know of. I don't have the acceleration of block 3.0kg needed to find the force using the information given. I'm assuming it'd be acceleration due to gravity at an angle perpendicular to the inclined plane but I'm not sure.
  2. jcsd
  3. Jun 19, 2011 #2
    Where did you get that problem from?

    There is a misconception in it I think... if I try and figure out what it is supposed to be, there is like 5 different possibilities.

    The problem is that force vectors are supposed to be drawn like this:

    see how all of the force vectors originate AT the object the force is applied to and move out? You don't just have naked force vectors like the one in your diagram.

    I don't know if it's the net force or an external, and in both cases we still don't know if the force is applied to the 4.8kg block system, or the 1.8kg block.

    I am guessing that the most likely case is that it is an external force being applied to the 1.8kg block only. which should instead be drawn such that the beginning of the vector is in the center of the 1.8kg block.

    Then you will have the 29N force on the 1.8kg block along with the component of the gravitational force on the 1.8kg block plus the force of the 3kg block pushing it down.

    Then just exploit the fact that all blocks will have the same acceleration (the 3kg block, the 1.8kg block, and the 4.8kg block system) to find the net force on the 3kg block. but that includes gravity, so subtract the force of gravity that was acting on it and what you are left with is the force of the 1.8kg block acting on the 3kg block.
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