# Why does Fn = mgsinΘ

1. Dec 19, 2011

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.

2. Dec 19, 2011

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

edit: yeah just read the sticky. my bad >_<

3. Dec 20, 2011

### 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.

4. Dec 20, 2011

### 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