# Mass of air bubble

Why is the mass of air bubble in material medium considered to be negative?

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berkeman
Mentor
Why is the mass of air bubble in material medium considered to be negative?
Please provide a link where this is used. Negative mass for a bubble would seem to be a clumsy computational tool.

DaveC426913
Gold Member
Why is the mass of air bubble in material medium considered to be negative?
Well, at first glance: the air bubble is going to be bouyant, so yeah, a self-contained discrete object (and no skin, like a balloon has) with, essentially, a negative mass. If the material is sensitive to strain, the bubble would push upward.

$$F=mg-\frac{m}{\rho_{air}}\rho_{water}g= (m(1-\frac{\rho_{water}}{\rho_{air}}))g$$
so that the mass gets modified to:
$$m \rightarrow m(1-\frac{\rho_{water}}{\rho_{air}})$$
which can be negative?

I think what's more common is to define an effective gravity rather than an effective mass, and have:
$$g \rightarrow g(1-\frac{\rho_{water}}{\rho_{air}})$$