Calculating Coefficient of Kinetic Friction for Inclined Plane Problem

[SanityMan]

Okay, here is the problem I am working on...

A block of a certain weight(w2) is sliding down an inclined plane at angle theta. There is a plank on top of the block with a certain weight (w2) that is attacked to a cord at the top of the plane so that it does not move.

I need to find the coefficient of kinetic friction in terms of the masses of each block and the angle theta. The coeffecient is the same between both surface contacts.

I am using Newton's 2nd law and I'm stuck with the friction. In my free body diagram I am not sure which direction the force of friction is going between the block and the plank nor how it affects both.

 

[cepheid]

Since you're trying to figure out directions, hopefully you defined a coordinate system first. After you've done that, think about this: in what direction is the block trying to slide past the plank? Friction always opposes this motion Likewise for the block sliding down the inclined plane.

 

[Nenad]

Its better to say that Friction is a force that opposes sliding.