Bubble problem---momentum Here is the problem I'm having trouble with: Some solidified lava contains a pattern of horiznotal bubble layers separated vertically with few intermediate bubbles. As the lava was cooling, bubbles rising from the bottom of the lava separated into these layers and then were locked into place when the lava solidified. The rising bubbles quickly become sorted into layers. The bubbles trapped within a layer rise at speed Vt = 0.5 cm/s. Bubbles breaking free from the top of one layer rise to join the bottom of the next layer. The rate at which a layer loses height at its top is dy/dt = vf = 1 cm/s. What are the speed and direction of motion of the layer's center of mass? To be honest, I wasn't really sure where to start with this problem. I know the answer (the center of mass moves 1.5 cm downward, and the bubbles rise but the layers descent), but I don't know how to get there.