# Is there any pressure inside the free falling fluid?

• sbu
In summary: Since pressure is continuous at the interface between the fluid and the air, the pressure would be read as 0 at the interface.
sbu
Imagine a high cylindrical vessel (like a vertical pipe with bottom installed) of an OD = 10 inches and fluid inside that vessel (e.g. water). If we install pressure sensor on the wall of the vessel (inside the the vessel) near the bottom, it will read hydro-static pressure of the fluid column inside the vessel.

Now imagine that we will take the bottom of the vessel away, so that fluid will fall off freely, but we will keep filling the top of the vessel with fluid so that it will be constantly kept full, even though the fluid is just leaking out at the bottom. What will the pressure sensor read?

The versions that were brought up during discussion with my colleagues are:
- sensor will read zero pressure because the fluid is in free fall and hence it is a condition similar to the absence of gravity (so falling fluid will have no weight and hence no hydro-static pressure will be exerted on the sensor)
- sensor will read hydro-static pressure minus friction pressure loss between the fluid and the walls of a vessel

It will be interesting to hear your thoughts on this :)

This seems interesting and I've been thinking of it for a while, but I think I have convinced myself, the fundamental principle of hydrostatics states that the change in pressure ΔP = pgΔh, for a free falling object (assuming negligeable tidal forces), the object is "weightless", it will seem at it is in outer space [this is Einstein's equivalence principle], so g = 0 and ΔP = 0, but there must be some constant pressure out there, it might due to friction, one thing for sure that the sensor will read a constant pressure that isn't issued from the fluid itself, but transmitted by the fluid [Pascal's theorem], if the sensor read only relative pressure, the result will be zero (+ some errors due the bumbing with air outside at it's pressure get's higher), I'd like to hear some thoughts too !,

There is a simple way of reasoning out the answer to this question. Here are two questions to stimulate your thinking:

1. What is the pressure in the air in the region immediately surrounding the fluid exiting from the bottom of the pipe?

2. Is pressure continuous at the interface between the fluid and the air?

Chet

Andy Resnick

## 1. What is "free falling fluid"?

"Free falling fluid" refers to any fluid that is in a state of free fall, meaning it is not under the influence of any external forces such as gravity or air resistance. This can occur in situations such as a fluid being dropped from a height or in outer space.

## 2. Is there any pressure inside free falling fluid?

Yes, there is still pressure inside free falling fluid. This is due to the fluid's own weight and the forces of surface tension and viscosity acting on it.

## 3. How does pressure change in free falling fluid?

The pressure in free falling fluid decreases as the fluid falls, as the weight of the fluid decreases due to gravity. However, the pressure may increase again if the fluid encounters resistance or changes direction.

## 4. Why is it important to study pressure in free falling fluid?

Understanding pressure in free falling fluid is important in various fields such as fluid mechanics, aerodynamics, and astrophysics. It allows us to predict the behavior of fluids in different environments and can also help in the design of structures and vehicles that operate in or through fluids.

## 5. How is pressure inside free falling fluid measured?

Pressure inside free falling fluid can be measured using instruments such as pressure gauges, manometers, and barometers. These instruments use principles such as hydrostatic equilibrium and Boyle's law to determine the pressure of the fluid at a specific point or over a certain distance.

• Mechanics
Replies
4
Views
2K
• Mechanics
Replies
10
Views
2K
• Mechanics
Replies
1
Views
1K
• Mechanics
Replies
1
Views
2K
• Mechanics
Replies
4
Views
2K
• Mechanics
Replies
12
Views
2K
• Mechanics
Replies
13
Views
2K
• Mechanics
Replies
12
Views
2K
• General Engineering
Replies
5
Views
3K
• Mechanics
Replies
1
Views
2K