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Homework Help: Stuck in a problem

  1. Mar 11, 2012 #1
    attachment.php?attachmentid=44953&stc=1&d=1331458110.jpg

    q) A force F acts on a block of mass m placed on a horizontal smooth surface at an angle Θ with horizontal. Then

    (A) If F sinΘ < mg then a = (F + mg) / m

    (B) Acceleration = FcosΘ/m where, F > mg cosecΘ

    (C) Acceleration = F/m if FsinΘ > mg

    (D) If F SinΘ > mg then a = (F + mg) / m

    my work
    --------

    on drawing the F.B.D we we get..

    clearly there will be linear and vertical acceleration as well...
    Now if F sinΘ > mg then the Normal force will be 0
    => F sinΘ - mg = may ---- (i)

    and, F cosΘ = max ------ (ii)

    Adding the two eqns. we get,

    F sinΘ + F cosΘ - mg = m (ax + ay)

    or, ax + ay = (F - mg) / m

    but the correct option is d.

    couldn't represent vectors with their notations so please understand
    it urselves.

    Am i wrong? if yes then where and why?

    Thanks in advance...!!
     

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  3. Mar 11, 2012 #2

    Doc Al

    User Avatar

    Staff: Mentor

    None of those answers look right. Are you sure there's not a typo in there? (Check the orientation of the > signs.)

    And you can't just add perpendicular vector components, such as ax + ay.
     
  4. Mar 11, 2012 #3
    why not....?????
     
  5. Mar 11, 2012 #4

    Doc Al

    User Avatar

    Staff: Mentor

    They are vectors. Direction counts.
     
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