# why is mass of air bubble in material medium considered to be negative?

The mass of air bubble in any medium is considered as negative. Is the air bubble is massless. I m in confusion. can we not neglect the mass of air bubble in material medium. But i have found in many books the mass of air bubble in material medium as negative. please discuss.....

• "The mass of air bubble in any medium is considered as negative" This is a calculating convenience, not a case of actual negative mass. – dmckee --- ex-moderator kitten Jul 14 '13 at 13:27

## 1 Answer

In these discussion, by "mass" they mean $$m=\int dV (\rho - \rho_0)$$ where $\rho$ is the actual density in the volume $dV$ – effectively zero for an air bubble – and $\rho_0$ is the normal density of the fluid. So if there's just the fluid, we call it zero. Consequently, an air bubble (more precisely vacuum bubble...) is assigned the mass equal to the minus mass of the liquid that is needed to fill the bubble.

This negative mass is heuristically OK to explain why the bubbles go up, and so on. But of course, in normal definitions of the mass, the mass is never negative.