Pressure vs. weight

[maxmanta]

Since water cannot be compressed, isn't water "pressure" a contradiction?

The density of gasses is variable and so "atmospheric" pressure (like in your air tank or recompression chamber) applies. But when we speak of water "pressure," aren't we talking about the WEIGHT (and DOWNWARD force) of the water column?

 

[jeffsterinsf]

You are correct in that the weight of the column of water is related to the pressure at a given depth. There is water pressure in the pipes in your house. That pressure works against the weight of the water in the pipes so that the water will flow up to your shower head, or second floor.

"Weight" is a manifestation of gravity and the mass of an object. It is what a scale measures -- the downward force.

"Pressure" is the force exerted on a surface, in this context from the water or gas. Just because a fluid is (nearly) incompressible, doesn't mean that it isn't exerting pressure.

Pressure, however, acts in all directions in a fluid (liquid or gas).

 

[maxmanta]

jeffsterinsf once bubbled...
You are correct in that the weight of the column of water is related to the pressure at a given depth. There is water pressure in the pipes in your house. That pressure works against the weight of the water in the pipes so that the water will flow up to your shower head, or second floor.

"Weight" is a manifestation of gravity and the mass of an object. It is what a scale measures -- the downward force.

"Pressure" is the force exerted on a surface, in this context from the water or gas. Just because a fluid is (nearly) incompressible, doesn't mean that it isn't exerting pressure.

Pressure, however, acts in all directions in a fluid (liquid or gas).

Thank you! That helps alot!

 

[donacheson]

maxmanta once bubbled...
Since water cannot be compressed, isn't water "pressure" a contradiction?

The density of gasses is variable and so "atmospheric" pressure (like in your air tank or recompression chamber) applies. But when we speak of water "pressure," aren't we talking about the WEIGHT (and DOWNWARD force) of the water column?

In addition to what jeff... had to say, the concepts of weight and pressure are different in another respect. Weight has dimensions of force, e.g. pounds; pressure has dimensions of force per unit area, e.g. pounds per square inch.

No material is truly incompressible, but that's not the critical issue. All fluids, both liquids and gases exert pressure equally in all directions. That isn't true of solids. For example, a column supporting a deck exerts a lot of pressure (and weight) on the footing beneath it, but little pressure on the air surrounding its sides.

 

[Dr Deco]

Dear maxmanta:

Pressure

Many readers answered your question, so I will add only a bit more. Pressure is what you would feel if you thumb was pressed against the end of a hose with either pressurized air or pressurized water in it. If you had a “calibrated thumb,” you would be able to tell the pressure in the tube. You could also tell the pressure in an auto tire by holding your thumb on the stem. Compressibility does not influence pressure. If your fingers were aught in a hydraulic jack, you would not care whether it was driven by compressed air or compressed oil.

Air Pressure

Air pressure, as we understand it was a concept developed in the early 1600s by scientists such as Torricelli (inventor of the mercury barometer), and Pascal. It was the latter that stated, “We live at the bottom of an ocean of air.”

Dr Deco :doctor:

 

[donacheson]

Dr Deco once bubbled...
Dear maxmanta:

Pressure

Many readers answered your question, so I will add only a bit more. Pressure is what you would feel if you thumb was pressed against the end of a hose with either pressurized air or pressurized water in it. If you had a “calibrated thumb,” you would be able to tell the pressure in the tube. You could also tell the pressure in an auto tire by holding your thumb on the stem. Compressibility does not influence pressure. If your fingers were aught in a hydraulic jack, you would not care whether it was driven by compressed air or compressed oil.

Dr Deco :doctor:

I'm going to nit-pick you a bit, Doc. If you pushed your thumb against a pressurized hose, you'd feel probably feel both pressure and total force (weight). My own impression is that one's skin senses pressure but the muscles in your hand and arm sense total force.

 

[Aquamaniac]

donacheson once bubbled...


I'm going to nit-pick you a bit, Doc. If you pushed your thumb against a pressurized hose, you'd feel probably feel both pressure and total force (weight). My own impression is that one's skin senses pressure but the muscles in your hand and arm sense total force.

\\

Yeah good point don, that's a very relavant point and very related to the post.

 

[Dr Deco]

Dear Don:

Pressure vs. weight

In many aspects, these are one and the same. We generally think of weight as applied to a solid or liquid that you can hold in your hand or set on a balance (or scale). Pressure is that force when the fluid is contained.

Air Pressure

Air pressure can be spoken of as both. We speak of “air pressure” but in reality it is the weight of the air extending up several miles. Possibly the easiest way to separate the two is to imagine what we would experience in a gravity-free environment, such as on the space station. Pressure of air in a hose could be experienced, but weight would not be. (Mass and inertia would still be present, of course.)

Dr Deco :doctor:

 

[jonnythan]

The reason we speak of water pressure at depth is because the pressure is a force per unit area. At a given depth, the *pressure* is equal on all surfaces, no matter how large they are.

However, the *force* or *weight* present depends on the size of the surface. To get force, you just multiply pressure times the area of the surface you're looking at.

But when we speak of water "pressure," aren't we talking about the WEIGHT (and DOWNWARD force) of the water column?

That's exactly right.. the pressure comes from the weight and force of the water above you. Just remember that pressure is constant at a depth exerted *on* a surface (such as your skin, or a tank, or the surfaces of your drysuit). Essentially, when vertical, the top of your head is feeling the *weight* of all the water above it. This may be 14.7 psi (just the water, not including the atmosphere), or about 500 pounds for a 6 inch by 6 inch head. Obviously, the fact that the pressure is just about equal in all directions around your head, and that it's filled with incompressible fluids and gases at the same pressure mean it won't collapse, but...

Still kinda cool to think that the top of your head can essentially be holding up hundreds of pounds of water, eh?

 

[donacheson]

Dr Deco once bubbled...
Dear Don:

Pressure vs. weight

In many aspects, these are one and the same. We generally think of weight as applied to a solid or liquid that you can hold in your hand or set on a balance (or scale). Pressure is that force when the fluid is contained.

Air Pressure

Air pressure can be spoken of as both. We speak of “air pressure” but in reality it is the weight of the air extending up several miles. Possibly the easiest way to separate the two is to imagine what we would experience in a gravity-free environment, such as on the space station. Pressure of air in a hose could be experienced, but weight would not be. (Mass and inertia would still be present, of course.)

Dr Deco :doctor:

If by "air pressure" you mean atmospheric pressure, yes, it's the weight of the atmosphere per unit area. If by "air pressure" you mean the pressure insde a scuba tank, then what does weight have to do with it? In fact, air pressure has meaning without invoking the concept of weight.

Let's keep this clear: weight is a force exerted by gravity; pressure is a force per unit area, whether that force is exerted by (as a consequence of) gravity or whatever.