Topic: Got some questions for the engineers and physicists......

[J.Webster Designs]

OK guys, I am having a hard time understanding some concepts pertaining to Pressure and Force.... My Fiance is an engineering Graduate student at University of Florida and She was trying to explain it all to me... it was a complete failure as she couldn't really explain it in a way that I understood.

Here is the question.

You have Two different size tanks (one small, one large) you can pick a size for your calculations as it doesn't matter to me. you have a compressor of the same horsepower and same CFM rating. The Compressor will be hooked up to each tank SEPARATELY.  you fill the Small tank to 50 PSI and you fill the large tank to 50 PSI. 

SO we have a small tank filled to 50 PSI and we have a large tank filled to 50 PSI as well.

The question is:

IS the FORCE on the walls of the tank equal in the two tanks... OR is the FORCE on the walls of the tank GREATER on the LARGE tank and LOWER on the small tank?

I have other questions but we will see where this one goes first.

[Markcb750]

The force per unit area is the same.  

The total force for the larger tank will be greater proportional to the internal area of the two tanks.

Your question empathizes FORCE, force in a hydraulic/pneumatic system is derived from pressure, which is expressed in force per area.  equal pressures against different areas will generate different forces.  the master cylinder and the caliper piston on the SOHC four front brake are at the same pressure but the force is much greater against the brake disk then your fingers.

[myhondas]

If you think about your question, you answered yourself.  50 Pounds per Square Inch(PSI) in both containers is the force on the walls.....the only difference between the two is the volume. If you opened a small valve on both boxes, you would feel/read the same 50 PSI, but you would have the air escaping for a lot longer in the big box compared to the small one.

[Markcb750]

Interesting, you think he is just asking if the air pressure is the same in both tanks?


I thought he was interested in the force each tank had to contain, which is proportional to its area.

[myhondas]

Well that is the answer   FORCE = PRESSURE     SIZE=VOLUME.....   his question was FORCE on the walls.....

[BobbyR]

I think he is asking if the the walls in the larger tank would need to be stronger due to the larger surface area of the tank walls. I think that is the crux of his question, I could be wrong.

[J.Webster Designs]

The pressure in both of the tanks is equal. 50 PSI in both tanks.

I am just wondering if the force on the walls of the two different tanks is the same.

My thought process is that if the pressure in the two tanks is the same, the force on the walls of the tank should be the same no matter the size of the tank? The only thing changing between the two tanks is the volume that it can hold. so why would the forces on the walls of the two tanks be different if all else is equal save for the volume?

[Markcb750]

Well that is the answer   FORCE = PRESSURE     SIZE=VOLUME.....   his question was FORCE on the walls.....

Well not exactly.

Force does not equal pressure.  

Force in physics and engineering is the influence that produces a change in a physical quantity; pressure = force/area

Pressure is an effect which occurs when a force is applied on a surface.

Force is a fundamental measure

Pressure is a combination of two fundamental items in this case lbs and length^2.



size is a generic term applied to force, length, weight, or volume.

[Markcb750]

The pressure in both of the tanks is equal. 50 PSI in both tanks.

I am just wondering if the force on the walls of the two different tanks is the same.

My thought process is that if the pressure in the two tanks is the same, the force on the walls of the tank should be the same no matter the size of the tank? The only thing changing between the two tanks is the volume that it can hold. so why would the forces on the walls of the two tanks be different if all else is equal save for the volume?

Do you understand what I am telling you about the relationship between pressure and force?

Think about the area of the inside surface of the two tanks, if the volume is larger won't the area be larger?

[J.Webster Designs]

I understand that pressure = force/area.

I agree that in order to get a larger volume you have to have a larger area of cylinder walls.

I just cant see the relationship for some reason. I am just trying to understand this concept as my entire life i have assumed that the force on the walls of the tank would be the same.

Let me try to explain my thought process to see if you can help me figure out where my problem with understanding this lies.

same scenario. two tanks equal pressure inside. same wall thickness on the tanks. basically all else equal in the most literal sense except for the volume of the tank.

I am putting the same amount of pressure into these tanks. so why would a change in volume make a difference in anything except for holding more air?

[sangyo soichiro]

Midnight, answer me this:

1. The force per area (pressure) is the same in both tanks.

2. One tank has a larger surface area than the other.

What does that imply?  Think it through.



Or try this example:
You have two windows.  One has twice the surface area as the other.  Let's say we sucked all the air out of the house (play along with me here...).  The atmosphere exerts 14.7 PSI (pounds per square inch).

Although both windows have the same pressure exerted on them (14.7 PSI), which window has the most force exerted on it?  And how much greater is that force than the force on the other window?

[sangyo soichiro]

Or try this example:

You have two big-buck football players.  You have two of those football push sleds. 

One sled has only one push pad and can only be used by one football player at a time.  The other sled has two.  Each football player can exert, say, 350 lbs of force on the pad.

One sled has twice the area as the other (it has two pads, while the other only has one).



Which sled has the most force exerted on it?

[BlindJoe]

what's easier to push through a piece of paper, a pencil or a can?

[J.Webster Designs]

OK.... i finally figured it out thanks to my fiance finally listening to my flawed logic... and it was because there was a disconnect between engineers knowing the difference between force per square inch and total force of the tank.

Because I have grown up thinking that pressure was a force I was interchanging the terms not knowing that it was a false way of thinking.

In my head, when i was being told the the force would have to increase in the big tank, that it was the force per square inch. when in fact the force that would have to increase is the total force of the tank.

Thanks for trying to help me with this.

(BTW... she is a TA ) HAHAHAHA

[Markcb750]

* I posted this while you where learning from the TA, hope it helps to reinforce the concept.

If you assume the larger tank has an area of 3000 square inches, and the smaller tank an area of 30 square inches.
Using the pressure = force/area formula the total force the larger tank must withstand in its skin is 150,000 pounds, the smaller tank's skin must withstand 1500 lbs.


The shape of the tank can be a significant factor in designing a pressure vessel.

Lets assume a round tank.

Think of the tank as a series of hoops held together.  Each hoop 1 inch wide.  for every one inch traveled along the hoop 50 pounds of force is pushing against the skin.

In the small tank lets assume each hoop has a length of 15 inches; each hoop must retain 750 pounds. This force is withstood by the skin and if divided by the area of the cross section of the hoop it is stated as "hoop stress" and expressed in pounds per square inch.

Increasing the volume:

If you keep the diameter of the tank the same and just make it longer the hoop stress in the wall will be virtually the same, so to hold 100 times more air you can just make your tank 100 times longer. It will not fail with the same skin thickness because the 148,500 pounds of additional force will be distributed.


If you make your larger tank the same length a 100 times increase in volume will require it to be 10 times larger in diameter.  In this case the skin will need to be thicker because the hoop stress will be much higher.  Each one inch wide hoop will be 10 times longer, 150 inches, and subjected to 7500 pounds.  The hoop stress will be correspondingly higher.



The numbers I am using for length of hoops is not derived from my stated area changes.

(For you sticklers; I do not want to add confusion so I am omitting the bending induced stress for this explanation.)

[J.Webster Designs]

Mark.... Thank you for your added reinforcement.... That is just about the same way that we arrived at my ultimate understanding.

Thanks for taking the time to help a fellow member out when I was confusing terms. I know that term confusion can infinitly increase the confusion when a person is asking for something different than what they are ACTUALLY asking for....

BTW... You ultimately answered my question within the relm of this thread about 50 times faster than me and the "TA" finally reached the conclusion. So props to you!

Cheers,
Justin

[Markcb750]

Your welcome,  If you know a little about electricity pressure and Voltage are analogous.