Fluid Forces in a Manometer: Understanding Pressure and Kinetic Energy

AI Thread Summary
The discussion focuses on understanding fluid forces in a manometer, particularly in two cases. In Case 1, it is established that atmospheric pressure exerts a downward force on the fluid, which must exert an equal upward force due to Newton's law of action-reaction. Case 2 introduces a gas source with higher kinetic energy, raising questions about how this pressure affects the liquid in the manometer. The response suggests practical experimentation with a homemade manometer to visualize the effects of pressure changes. A foundational understanding of pressure in fluids is recommended before delving deeper into fluid mechanics concepts.
DumpmeAdrenaline
Messages
80
Reaction score
2
Thread moved from the technical forums to the schoolwork forums
Note: I am self-studying Material and Energy balance courses and I haven't done fluid mechanics yet.

Case 1) Consider the manometer in the figure below. Levels on both sides of the manometer which are open to the atmosphere are equal.
If we analyse the forces acting on the left side of the manometer. We have the atmospheric force exerting a downward force on the fluid and since the fluid is not undergoing acceleration it is either moving at a non zero speed or is stationary. Therefore the fluid must be exerting an equal but upward force on the atmosphere.

Case 2) If we hook up the left side of the manometer to a gas source containing higher kinetic energy molecules than the air molecules on the right side exposed to the atmosphere then those gas molecules are exerting a higher pressure. What would happen in this case?
Will the gas molecules push the liquid, bounce, pass through the liquid as bubbles?

I wrote case (1) mainly to link the upward force the fluid exerts and what is it a function of?
 

Attachments

  • P1.png
    P1.png
    1.3 KB · Views: 126
Physics news on Phys.org
DumpmeAdrenaline said:
Note: I am self-studying Material and Energy balance courses and I haven't done fluid mechanics yet.

Case 1) Consider the manometer in the figure below. Levels on both sides of the manometer which are open to the atmosphere are equal.
If we analyse the forces acting on the left side of the manometer. We have the atmospheric force exerting a downward force on the fluid and since the fluid is not undergoing acceleration it is either moving at a non zero speed or is stationary. Therefore the fluid must be exerting an equal but upward force on the atmosphere.
By Newton's law of action-reaction, the fluid exerts and equal but upward force on the atmosphere no matter what else is going on.
 
  • Like
Likes DumpmeAdrenaline
DumpmeAdrenaline said:
Case 1) Consider the manometer in the figure below. Levels on both sides of the manometer which are open to the atmosphere are equal.
If we analyse the forces acting on the left side of the manometer. We have the atmospheric force exerting a downward force on the fluid and since the fluid is not undergoing acceleration it is either moving at a non zero speed or is stationary. Therefore the fluid must be exerting an equal but upward force on the atmosphere.

Case 2) If we hook up the left side of the manometer to a gas source containing higher kinetic energy molecules than the air molecules on the right side exposed to the atmosphere then those gas molecules are exerting a higher pressure. What would happen in this case?
Will the gas molecules push the liquid, bounce, pass through the liquid as bubbles?

I wrote case (1) mainly to link the upward force the fluid exerts and what is it a function of?
To answer your specific questions…

In case 1, the magnitude of the force depends on the value of atmospheric pressure and the area of the liquid’s surface. The value of the atmospheric pressure depends on the average speed and mass of the air molecules and their number-density (how many molecules there are per cubic metre).

For case 2, I recommend you get a length of (clean!) clear plastic tube with some water. You can hold the tube in a U-shape and you have made your own manometer!

Increase the pressure on one side by gently and briefly blowing into one end. Then quickly slide slide your finger over the end in your mouth (yes it can be done!) to trap the extra gas. You will see what happens.

Similarly, you can reduce the pressure on one side by briefly and gently sucking on one end. Avoid sucking too hard and choking!

It sounds like you may have to take a step back and first learn about the basics about pressure in gases and liquids.

By the way, liquids and gases are often both referred to as ‘fluids’. So here it is best to say that the atmosphere ex erts a force on the liquid (and vice versa).
 
  • Like
Likes DumpmeAdrenaline
Thread 'Collision of a bullet on a rod-string system: query'
In this question, I have a question. I am NOT trying to solve it, but it is just a conceptual question. Consider the point on the rod, which connects the string and the rod. My question: just before and after the collision, is ANGULAR momentum CONSERVED about this point? Lets call the point which connects the string and rod as P. Why am I asking this? : it is clear from the scenario that the point of concern, which connects the string and the rod, moves in a circular path due to the string...
Back
Top