Subject: Re: OT: a fifth grade physics question
k3: Why does water care if the object is going to sink or float?

sano: Specific gravity.

Or relative density.
https://en.wikipedia.org/wiki/...
Relative density can also help to quantify the buoyancy of a substance in a fluid or gas

Perhaps more to the point, buoyancy.
https://en.wikipedia.org/wiki/...
an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object.