What is a fluid?

[Vman455]

Breaking this off so as not to muck up another person's thread. In that thread, I wrote:

I'm working through White's Fluid Mechanics this summer, and the first chapter starts with a discussion of one deceptively simple question: What is the difference between a solid and a fluid? The author points out that most laypeople, even though they "know" what makes the two states different, can't actually define it.

And another poster responded:

Fluids flow and conform to a container, solids (mostly) maintain their shape, barring sufficient outside forces. Why is that difficult?

But...

"Some apparently 'solid' substances such as asphalt and lead...actually deform slowly and exhibit definite fluid behavior over long periods." (White, Frank M. Fluid Mechanics, 8th ed. New York: McGraw Hill, 2017. 4)

(Ellipses where I removed the actual answer).

Solids (mostly) maintaining their shapes. Glass, over extremely long time periods, flows. But glass is a solid. Thus, the ability to flow and conform to the shape of a container is not the distinction. So, what makes lead, asphalt, or glass solids vs. fluids...?

I'll post the answer in this thread tonight if no one guesses it before then.

[Shaneajanderson]

Originally Posted by Vman455

Breaking this off so as not to muck up another person's thread. In that thread, I wrote:



And another poster responded:



But...

"Some apparently 'solid' substances such as asphalt and lead...actually deform slowly and exhibit definite fluid behavior over long periods." (White, Frank M. Fluid Mechanics, 8th ed. New York: McGraw Hill, 2017. 4)

(Ellipses where I removed the actual answer).

Solids (mostly) maintaining their shapes. Glass, over extremely long time periods, flows. But glass is a solid. Thus, the ability to flow and conform to the shape of a container is not the distinction. So, what makes lead, asphalt, or glass solids vs. fluids...?

I'll post the answer in this thread tonight if no one guesses it before then.

This is why I put the (mostly).

As far as the difference, in fluids the individual molecules move freely, but in a solid they are constrained by those around them. The 'flowing' in lead, asphalt, glass, and other such solids is do to shifting crystal structure, but the individual molecules hold their positions relative to each other, outside of this 'sliding' if you wish to call it that.

Hope that makes sense, I'm good with understanding, bad with wording.

[redpoint5]

I've not heard that lead loses its shape or will conform to a container under STP. I have heard that glass is not a liquid and does not flow over time (under STP).

I would guess that a solid maintains the majority of the atomic bonds to neighboring atoms, while a liquid does not. The heat energy is insufficient to break the atomic bonds even under the influence of gravity.

[Stubby79]

A liquid has (molecular?) bonds, unlike a gas, but these bonds are not rigid as in a solid, so while able to move around, they (want to) stick together(unless forced apart).

[Angel And The Wolf]

Originally Posted by Vman455

Glass, over extremely long time periods, flows. But glass is a solid.

glass is called a "super cooled liquid"

From https://www.reference.com/science/gl...dd009b92fd2973

"True solids form crystalline structures that lock molecules into place. They retain their shape unless the temperature increases to above their melting point. However, glass, amber and plastic do not form such solids when their liquid form cools. While glass is more rigid than liquid, it is not as ordered as crystalline solids."

[Vman455]

Originally Posted by Angel And The Wolf

glass is called a "super cooled liquid"

From https://www.reference.com/science/gl...dd009b92fd2973

"True solids form crystalline structures that lock molecules into place. They retain their shape unless the temperature increases to above their melting point. However, glass, amber and plastic do not form such solids when their liquid form cools. While glass is more rigid than liquid, it is not as ordered as crystalline solids."

Exactly; chemists classify glass as an "amorphous solid."

Originally Posted by Shaneajanderson

This is why I put the (mostly).

As far as the difference, in fluids the individual molecules move freely, but in a solid they are constrained by those around them. The 'flowing' in lead, asphalt, glass, and other such solids is do to shifting crystal structure, but the individual molecules hold their positions relative to each other, outside of this 'sliding' if you wish to call it that.

Hope that makes sense, I'm good with understanding, bad with wording.

You're so close, I'm going to give it to you! Here's the technical definition:

"A solid can resist a shear stress by a static deflection; a fluid cannot." (Emphasis in original; that's how fundamental this definition is to the study of fluid mechanics!).

Anyone else got any good physics riddles?

[Shaneajanderson]

Originally Posted by Vman455

Exactly; chemists classify glass as an "amorphous solid."



You're so close, I'm going to give it to you! Here's the technical definition:

"A solid can resist a shear stress by a static deflection; a fluid cannot." (Emphasis in original; that's how fundamental this definition is to the study of fluid mechanics!).

Anyone else got any good physics riddles?

Not bad for a dropout hick eh?