# Relative density in a fluid

This is a bit of a thing I thought about today that might be complete malarky but it'd help me learn as to why it's a logical fallacy as it may well be.

Oh, and I apologize if the title is misleading, since I know see "Relative density" has a scientific definition.

Water has a definite density. Lakes have a density that is more-or-less continuous throughout its body. Let's say that the continuous density of lakewater is the density of water, $1\ g/cm^3$. Density can be loosely defined as the amount of mass in an amount of space of a substance, and arguably describable as the amount atoms in a given volume. If I were to swim in such a lake and put my head underwater, the density of the fluid I'd be in relative to me would be the density of the water.

However, here's the caveat: what if, say, I was now moving fast in the water. Atoms of water are hitting my face at a much higher rate than they were when my head was still. Relative to me, what I could be experiencing is comparable to having my head still in a volume of lakewater with a higher density.

There's an amount of particles hitting or at least touching or interacting in some way with my face when my head is still, and density should be proportional to how much it's doing that. If the rate that which this is happening is increased, relative to me, shouldn't this lake have an increased relative density?

This may sound ridiculous, but I'm interested in expanding my learning, and I like taking stabs at things to see why it's wrong to help me understand.

• Regardless of what your head is experiencing, the mass of water (assuming it's incompressible) in a given volume of it will remain the same. I think you're making a hasty generalization when you say that because your interaction with the water at a higher velocity is similar to that at a higher density, the two must be related. – user122423 Jun 12 '17 at 4:31
• I took away the general relativity tag, since this has nothing to do with general relativity. – magma Jun 12 '17 at 8:23
• "There's an amount of particles hitting or at least touching or interacting in some way with my face when my head is still, and density should be proportional to how much it's doing that." No, that's fluid pressure. And dynamic fluid pressure takes into account your movement speed. Density is unaffected. – MrBrushy Jun 12 '17 at 9:48
• Water is basically incompressible. We usually assume the density doesn't change noticeably. – JMac Jun 12 '17 at 9:50
• You're confusing density with pressure, or more precisely, compressive stress. – Chester Miller Jun 12 '17 at 15:18