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Homework Help: Why does Fn = mgsinΘ

  1. Dec 19, 2011 #1
    There was a bonus word problem on my physics homework that i didnt know how to solve. its two masses, one on an incline plane connected tot he other hanging by a pulley. Heres a crudely drawn FBD of it.


    in the question Θ=37 m1=5kg and m2=6kg. assume no friction and find the acceleration and tension in the string. It says to use "special (picture of a triangle)" whats that mean.

    I looked up how to solve it and found that to find the normal force on an incline its Fn=mgsinΘ

    Can someone explain why this is? And I still havent solved it, but once i understand that part Ill try again before getting help.
  2. jcsd
  3. Dec 19, 2011 #2
    I think this might be the wrong section, sorry if it is

    edit: yeah just read the sticky. my bad >_<
  4. Dec 20, 2011 #3
    The normal force points perpendicularly to the surface; draw out the surface, the horizontal, the angle between them, the force of gravity and the normal force and try to use some geometry to get the [itex]\sin \theta [/itex].
    Alternatively I can never remember when to use [itex]\sin[/itex] or [itex]\cos[/itex] in these problems, I just think what would happen at [itex]0^o[/itex] and [itex]90^o[/itex] (Which angle would make the force disappear) to figure it out.
  5. Dec 20, 2011 #4

    Doc Al

    User Avatar

    Staff: Mentor

    This isn't true.

    To find the normal force, consider the force components on the mass perpendicular to the surface. What must they add to?

    You may find this helpful: Inclined Planes
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