Finding the Angle Phi in a Gravity-Affected System

[radagast_]

Hello.
I have the attached problem.
The body whose mass is m doesn't move relative to the body M. Them both are affected only by gravity.
I need to find the angle phi.

Can anyone please help me?

http://img135.imageshack.us/img135/4690/1113169793tx6.gif"

Thank you.

 

[Doc Al]

Show us what you've done so far and point out where you got stuck.

Hint: What's the motion of that contraption along the incline?

 

[radagast_]

Show us what you've done so far and point out where you got stuck.

Hint: What's the motion of that contraption along the incline?

well, I tried to draw the forces on the little mass m, and I know the whole contraption is under a force (mgsin(alpha)), and I got to the point that on the little mass, I have the tension and the mg force and a d'elambert force to the left.
Im kinda stuck on doing that on the whole thing. I don't really know how to relate to the little mass in reference with the M mass, ie - I want to write the motion equations from the accelerating contraptions' POV, but then I don't know how to take the m mass's forces (should I take it as another force, ie mg+Mg=mgsinalpha, or not...).

 

[Doc Al]

D'alembertian forces? Let's have none of that crazy talk!

What's the acceleration of the contraption (and thus mass m)? Analyze forces parallel and perpendicular to the incline and apply good old Newton.

 

[radagast_]

Thanks.
I think I got it... I'll try it and see how it turns out
:)