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Homework Help: Force needed to keep block from moving on frictionless triangular block

  1. Sep 26, 2008 #1
    1. The problem statement, all variables and given/known data
    This is straight from the book.

    A small block of mass m rests on the sloping side of a triangular block of mass M which itself rests on a horizontal table as shown in Fig. 4-50. Assuming all surfaces are frictionless, determine the force F that must be applied to M so that m remains in a fixed position relative to M (that is, m doesn't move on the incline).

    (I forgot to draw the surface the triangular block is on and sorry for the big picture!)

    The answer is:
    (m + M) [tex]\ast[/tex] g[tex]\ast[/tex] tan([tex]\phi[/tex])

    2. Relevant equations
    Force along the x-axis: m [tex]\ast[/tex] g [tex]\ast[/tex] sin([tex]\phi[/tex])
    Force along the y-axis: m [tex]\ast[/tex] g [tex]\ast[/tex] cos([tex]\phi[/tex])

    3. The attempt at a solution
    I know that there must be an equal and opposite force along the incline but I have no clue how to approach this problem.
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Sep 27, 2008 #2


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    Science Advisor
    Homework Helper

    Welcome to PF!

    Hi Strukus! Welcome to PF! :smile:
    No, that's the wrong approach …

    only one of those equations is correct …

    start again … you know the vertical acceleration of m is zero, so what is N (the normal reaction force ) …

    and then what is the horizontal component of N? :smile:
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