Static friction on an inclined plane.

[megatron99]

Homework Statement

http://img124.imageshack.us/img124/9997/problem3.jpg

Homework Equations

f=mgcos theta

The Attempt at a Solution

I am thinking the answer is f = mgcos theta because gravity only acts on y-component but the answer is f=mgsin theta . Can anyone explain?

 

[rl.bhat]

In the problem, f is the frictional force ,not the normal reaction.
Frictional force acts in the opposite direction of the motion of the block.The block moves down due to the component of weight along the inclined plane. What is that component?

 

[megatron99]

In the problem, f is the frictional force ,not the normal reaction.
Frictional force acts in the opposite direction of the motion of the block.The block moves down due to the component of weight along the inclined plane. What is that component?

x-component of weight?

 

[bleedblue1234]

Here's a revised pic...

This should help...

http://img18.imageshack.us/img18/4052/revisedpic.jpg