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This picture is from L.C.Epstein's book Thinking Physics. The upper can is filled with compressed air, and, when an opening is made on the right, the air comes out and the can shoots left. The question is what happens to the lower can, filled with vacuum, when we similarly make an opening. Does it move left - right - not at all?
Epstein says that the lower can doesn't move at all, "except for a momentarily slight oscillation about the center of mass". I'm not sure I understand this. The explanation is that the air incoming into the bottle provides force on the left inner wall to compensate for the lack of force on the opening, and this balances the force on the left outer wall from the outer air. Which seems convincing, but opens a path to more questions:
- Shouldn't the can still start moving from the moment we make the opening and until the air pressure inside the can is equalized with the outside air?
- If that in fact happens, why would it stop and return ("a momentary slight oscillation about the center of mass") and not simply continue moving right with the constant velocity it's acquired?