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Dynamics question

  1. Oct 9, 2013 #1
    1. The problem statement, all variables and given/known data
    a mass z is held on an incline plane with mass P. when the the masses are released, both objects begin to move and accelerate. find accell of the incline plane

    GIVEN: z,P,angle A, surfaces are frictionless

    solve in terms of these variables.
    2. Relevant equations

    f=ma

    3. The attempt at a solution
    i chose my positive x direction along the ground surface below the incline plane, toward the direction of its motion.

    so first i solved for the small mass on top the normal force, which is the force which applied onto the incline plane by the small mass.

    fnety=0=FNcosA - zg
    FN= zg/cosA

    now when i do the FBD of the incline plane the forces acting on it, are the normal (FN) which gives it an unbalanced force in the +X direction.

    so i get the expression

    fnetx= (z+P)a=FNsinA

    subbing in the FN,i get

    a= zgtanA/(z+P)

    is this correct? if not can you tell me where i am going wrong.
     
    Last edited: Oct 9, 2013
  2. jcsd
  3. Oct 9, 2013 #2
    The small mass accelerates along the y direction. So Fnet is not zero along this direction.
     
  4. Oct 10, 2013 #3
    Ahh I see..
    Fnety=zg=FN-zgcosA
    FN=2zg/cosA

    Ans; 2zgtanA/(z+P)
     
  5. Oct 10, 2013 #4
    The vertical acceleration of the small object is not g. It's not falling freely.
     
  6. Oct 10, 2013 #5
    would it make it more simple if i chose a different frame of reference.
     
  7. Oct 10, 2013 #6
    You could work in the non-inertial frame of the moving inclined plane. But I am not sure it makes simpler. Depends how it is easier to visualize for you. In the inertial frame attached to the ground or in the moving frame.
     
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