# How to find replacement function of a mass?

I wonder how I can find the replacement function of the center of the blue mass? The center of mass of the blue mass is $(0,0)$ and the blue mass is homogeneous. The masses do not move at t=0 in the system (Their initial speeds are zero). There is no friction in the system and the red mass is a point mass. $u$ is the angle of the red mass with $y$ axis.

I need help in learning the theory behind solving such systems. Thank you.

EDITED: I would like to show what I did till now to solve the problem. I followed the hints that Manishearth offered

$u=\alpha _0$ The definations of the blue mass center replacement:

$X_b(t)$ The replacement function that I try to fnd

$Y_b(t)$

The definations of the red mass Center replacement:

$X_r(t)$

$Y_r(t)$

1- Mass of center of entire system is in the beginning: $$r_{0x}=\frac{m. r. sin(\alpha _0)}{m+M}$$ $$r_{0y}=-\frac{m. r. cos(\alpha _0)}{m+M}$$ Mass center will not depend on time. If So,

$$mX_r(t)+MX_b(t)=(m+M)r_{0x}=m. r. sin(\alpha _0)$$ (1)

$$mY_r(t)+MY_b(t)=(m+M)r_{0y}=-m. r. cos(\alpha _0)$$ (2)

2-Energy is conserved

$$\frac{m (\dot X_r(t))^2 } {2}+ \frac{m (\dot Y_r(t))^2 } {2}+\frac{M (\dot X_b(t))^2 } {2}+\frac{M (\dot Y_b(t))^2 } {2} +mg(a+Y_r(t))+Mg(a+Y_b(t))=mg(a-r.cos(\alpha _0))+Mga$$ (3)

I dont know how to apply Hint 3 and Hint 4. Till now I have 4 unknown and 3 equations. Am I in correct way to solve $X_b(t)$ ? Thanks in advice

-
@Qmechanic: İt is not homework for any class. I am just self-learner in physics as engineer.(I thought the question myself because I try to learn to solving technics ) If I understand to solve such problems, I am going to clarify some points in my mind. Thanks for understanding. –  Mathlover Mar 11 '12 at 14:56
Have a look at the tag description. If you think that the tag does not apply, please edit it out of the question again. –  Qmechanic Mar 11 '12 at 15:12
I got it. Thanks for explanation. My question is for educational purpose in general. –  Mathlover Mar 11 '12 at 15:18
Also, could you elaborate on the meaning of "replacement function"? I think you're referring to the trick of using negative mass wile calculating center-of-mass, but I'm not sure. Google isn't helping =P –  Manishearth Mar 11 '12 at 15:37
Can you say a few words about your level of preparation. This could be a fairly difficult physics 101 problem or a fairly easy problem in Lagrangian mechanics (and looks chosen to highlight the advantages of using the Lagrangian approach). –  dmckee Mar 11 '12 at 16:19

Hint 4: There is circular motion going on in the frame of the blue body. Note that we can only apply our usual $m\omega^2 r$ formula in the moving frame of the body.