Why does Fn = mgsinΘ

  • Thread starter Esoremada
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  • #1
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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.

http://sketchtag.com/KS3pmhlzgq

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.
 

Answers and Replies

  • #2
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I think this might be the wrong section, sorry if it is


edit: yeah just read the sticky. my bad >_<
 
  • #3
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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.
 
  • #4
Doc Al
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I looked up how to solve it and found that to find the normal force on an incline its Fn=mgsinΘ
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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