# If KE is related to pressure on the wall for flowing liquid?

• hongiddong
In summary: This is a bit more complicated. If you look at a simple closed system, there is a force that opposes the flow (pressure in the smaller pipe), and the rate of flow is proportional to the pressure difference. In a more complex system (like the environment), there are other forces (e.g. viscosity) that can come into play that can influence the rate of flow. If you shrink the pipe, the pressure difference will increase, and the flow will slow down.
hongiddong
If there is no velocity and the fluid is moving still, then there is no kinetic energy, would there be no pressure exerted on the walls?

Or perhaps there is a basal level of kinetic energy of the molecules as they are vibrating?Lastly, how can there still be flow if the pressure gradient is constant everywhere? (meaning constant velocity).

Thank you physicsforum!

MisterX
Can you explain further, perhaps with an example? As a fluid mechanics guy, I have no idea what you are asking.

hongiddong said:
If there is no velocity and the fluid is moving still, then there is no kinetic energy, would there be no pressure exerted on the walls?

Or perhaps there is a basal level of kinetic energy of the molecules as they are vibrating?

I think you are misinterpreting what pressure means. Pressure (or rather, static or thermodynamic pressure) is really a measure of the mean kinetic energy of the molecules that make up a gas or liquid. This certainly still exists when a fluid is at rest. The only way to reduce this to zero pressure is to either remove all of the molecules (i.e. create a vacuum) or slow them down to zero velocity (the fluid would change state to a solid long before this would theoretically occur).

The bulk motion of the fluid is represented by the dynamic pressure, which is a continuum quantity and represents the kinetic energy per volume of a flowing fluid. That will certainly go to zero if the fluid slows down to zero flow. If you combine static pressure and dynamic pressure (and sometimes gravity) you get the total pressure for a flow, which pops up a lot in fluid mechanics and is the quantity that is conserved in Bernoulli's equation.

hongiddong said:
Lastly, how can there still be flow if the pressure gradient is constant everywhere? (meaning constant velocity).

You are misusing the term "pressure gradient". If the pressure gradient is constant, that means that the pressure is changing in space (e.g. along a length of pipe) at a constant rate, not that the pressure itself is constant everywhere. With a constant pressure gradient, there will be acceleration of the flow in the direction of decreasing pressure. If the pressure itself is constant, then there will be no acceleration (no force), and in the case of no viscosity, it will either remain still or else continue flowing however it already was. If viscosity is involved, then it is similar to friction and no pressure gradient means no flow, and the pressure gradient must be equal or greater to the viscous force in order for flow to be sustained.

hongiddong
hongiddong said:
If there is no velocity and the fluid is moving still, then there is no kinetic energy, would there be no pressure exerted on the walls?
Or perhaps there is a basal level of kinetic energy of the molecules as they are vibrating?
Taking a jab, ( Perhaps a difficulty in language translation and expression )
Are you asking about the kinetic theory of gases and the ideal gas law.?
And whether the microscopic average kinetic energy of a of a system of molecules is ever zero, so the macroscopic value expressed as pressure is ever zero?

hongiddong said:
Lastly, how can there still be flow if the pressure gradient is constant everywhere? (meaning constant velocity).

Edit/
I guess my post became moot while I was typing

Sorry guys for not communicating my thoughts properly. Thanks Boneh3ad for clearing this issue up! I feel better about this concept. Pheww physics can get intense.

I have two more questions that came up as these concepts are becoming more clear
1: in the example of Bernoulli's fluid moving from a big pipe going into a smaller pipe(assuming ideal conditions) if the fluid speeds up in the pipe,(therefore having more KE, why would the pressure be decreased in this situation).

2: In the same scenario, why is rate of flow(L/min) preserved as the fluid moves from a bigger pipe to a smaller pipe?
Is this because the fluid is incompressible and there is a constant force that is being applied to the fluid in the large pipe?

hongiddong said:
1: in the example of Bernoulli's fluid moving from a big pipe going into a smaller pipe(assuming ideal conditions) if the fluid speeds up in the pipe,(therefore having more KE, why would the pressure be decreased in this situation).

Again, you are mixing up pressures/energies. Bernoulli's equation deals with the bulk properties of a continuous fluid. Given its assumptions, the total pressure must be constant, so if the flow speeds up, the dynamic pressure increases, and the static pressure must decrease. It's essentially a transfer of that kinetic energy from random particle motion into a more organized direction, if you must think of it that way.

hongiddong said:
2: In the same scenario, why is rate of flow(L/min) preserved as the fluid moves from a bigger pipe to a smaller pipe?
Is this because the fluid is incompressible and there is a constant force that is being applied to the fluid in the large pipe?

Generally speaking, mass must be conserved. That means that in a closed system operating in steady state, there can't be any more mass entering than there is leaving the system. In the case where the fluid is incompressible, this means density is effectively constant and the volumetric flow rate going in equals that going out.

hongiddong

## 1. How is kinetic energy related to pressure on the wall for flowing liquid?

The kinetic energy of a flowing liquid is directly related to the pressure it exerts on the walls of its container. As the liquid moves, its particles collide with the walls, creating a force that results in pressure. The faster the liquid is flowing, the more kinetic energy it has and the higher the pressure it exerts on the walls.

## 2. Why is the relationship between kinetic energy and pressure important in fluid mechanics?

The relationship between kinetic energy and pressure is important in fluid mechanics because it helps us understand how fluids behave and how to control their movement. By understanding the role of kinetic energy in pressure, we can design systems that efficiently transport fluids and prevent potential issues such as leaks or bursts.

## 3. Can the kinetic energy of a flowing liquid be manipulated to control pressure?

Yes, the kinetic energy of a flowing liquid can be manipulated to control pressure. By changing the speed or direction of the flow, the kinetic energy and pressure can be altered. This is the principle behind devices such as valves or pumps, which are used to regulate the flow of fluids and control pressure.

## 4. Is there a limit to how much pressure a flowing liquid can exert on a wall?

Yes, there is a limit to how much pressure a flowing liquid can exert on a wall. This limit is known as the critical pressure, and it is the point at which the liquid becomes a gas or vapor. When this happens, the particles are no longer in close contact with the wall, reducing the pressure they exert. This limit is important to consider in the design and operation of systems involving high-pressure liquids.

## 5. How does the density of a liquid affect the pressure it exerts on a wall?

The density of a liquid is directly related to the pressure it exerts on a wall. The higher the density of a liquid, the more closely packed its particles are, resulting in a greater force and pressure on the wall. This is why liquids such as water, which have a higher density than gases, are often used in systems that require high levels of pressure.

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