# How much water to put in cup for maximum stability? [closed]

I found this problem in ‘200 puzzling physics problems’

1. An empty cylindrical beaker of mass 100 g, radius 30 mm and negligible wall thickness, has its center of gravity 100 mm above its base. To what depth should it be filled with water so as to make it as stable as possible?

Well, I solved it but want to understand the answer: why must the center of mass always lie on the water surface for maximum stability?

In the solution given in this book, it explains that the height of center of gravity would increase on filling more water after acquiring the condition of the center of mass lying on water surface but does not explain why the center of mass cannot be below it.

• Please note that the OP is not asking for the solution to the problem quoted at the top of the post. He is clearly asking a conceptual question: "why its center of mass always lie on the water surface for max stability". Thus this question should not be closed for the "homework-like exercise" reason. – PM 2Ring Sep 9 '19 at 13:28
• I agree with PM 2Ring. Should not be closed. – Bob D Sep 9 '19 at 13:33
• Well, is this question closed?? If yes, please tell me how to know if a question has been closed or not. – Prajwal Turkar Sep 9 '19 at 13:34
• It has not yet been closed, though there are two votes to close. PM 2Ring and I are arguing in favor of not closing the question. Hope that clarifies things. – Bob D Sep 9 '19 at 13:46
• @PM2Ring I agree that this question could be edited into a shape that's on topic. I don't think the current wording makes the bar, though. – Emilio Pisanty Sep 9 '19 at 14:16