Hello, The question I have is in regards to this page http://www.myphysicslab.com/molecule2.html where setting the gravity to zero and setting the mass of the red ball to 30 causes the system to move to the right slowly. You can reset the system after setting up the two variables so that no external forces are present. Why does the location of the two masses (system) change and how is this represented. Is there a conservation of momentum. I understand the part about solving for this system by finding the COM and then treating each ball as a separate spring problem like the next link http://www.myphysicslab.com/spring1.html, although I'm not sure what the spring constant becomes when solving for each system. Thanks. WHOOPS! nevermind. I think there is an error in the simulation. If you change the mass of the red ball to 90 and the gravity to 0 and reset the simulation and then change the red ball mass to 30, the system will shift to the right.