Solving Physics Problem: Why Fn = mgsinΘ?

[Esoremada]

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)" what's 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 haven't solved it, but once i understand that part Ill try again before getting help.

 

[Esoremada]

I think this might be the wrong section, sorry if it isedit: yeah just read the sticky. my bad >_<

 

[JHamm]

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 \sin \theta.
Alternatively I can never remember when to use \sin or \cos in these problems, I just think what would happen at 0^o and 90^o (Which angle would make the force disappear) to figure it out.

 

[Doc Al]

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