The nature of water pressure?

[Snowbear]

You guys need to go back to school.

I'd love to. My edumacation is lacking in certain impotant areas (like my brain). This was proved by the spinning sensation trying to follow this thread. Anyone care to cover my financial obligations at home while I'm there

 

[jonnythan]

Not disagreeing at all with what hes saying - just the way its said it.

Saying what a body experiences 'doesnt exist' or is 'imaginary' is not very scientific. There is an real inertial force in play with real conseqences. A better way of putting it -

Centrifugal force is actually not a force but the experience of an inertial force experienced in a rotating reference frame acting away from the center of the rotation. It is equal in magnitude but opposite to the centripetal force required to constrain the body to move in a circular motion.

I know, I know Im being an ass - but yould also be in a bad mood if some tosser just vandalised your car

Centrifugal force is the exact same experience as the feeling of being "pulled backward" when you're stepping on the accelerator of your car.

No one would claim that there's a force being exerted pulling them backward into the seat, just that they're being pushed foward.

It's absolutely identical, it's just inertial resistance to acceleration. In a rotating body, the acceleration is perpendicular to the direction of motion, and in a body accelerating linearly, it's in line with the direction of motion. There's no force.

 

[donacheson]

... it's just inertial resistance to acceleration. In a rotating body, the acceleration is perpendicular to the direction of motion, and in a body accelerating linearly, it's in line with the direction of motion. There's no force.

Yes, but it is VERY convenient to treat it as a force directed outward from the axis of rotation when a body is constrained (by gravity or a rope or friction or whatever) to follow a curved path.

There's some analogy between the balances of (1) centripital force (the product of centripital acceleration and mass) and centrifugal "force" when a body is in a stable orbit and (2) weight and buoyancy when a diver is "neutral". It's also very convenient to treat buoyancy as a single force described by Archimedes' Principle, while in fact it's actually the integral of water pressure over the surface of the diver and all his equipment.

Will that last paragraph provoke another path is this topic drift ? <G>

 

[jonnythan]

Yes, but it is VERY convenient to treat it as a force directed outward from the axis of rotation when a body is constrained (by gravity or a rope or friction or whatever) to follow a curved path.

In three years of studying engineering, I've done a lot of physics and a lot of force analysis. I've never seen any body drawn with or any problem calculated with a centrifugal force. It doesn't exist, and it doesn't factor into any equations. It's something a person thinks they feel because they don't understand acceleration when it's not in the same direction as motion.

Think about it this way.. you're cruising along a river in a tube with a rope tied to it. Someone yanks on the rope with a continuous force. The instant the force begins, you're for all practical purposes a rotating body on the rope.. you'll be feeling your "centrifugal" force. But since you're not moving fast enough to actually rotate around the center point, you follow a curved path toward the direction of pull.

At what point does the "centrifugal force" just get replaced by "intertia"?

No one uses "centrifugal force" as a "convienent" way to calculate forces or acceleration.

 

[donacheson]

... No one uses "centrifugal force" as a "convienent" way to calculate forces or acceleration.

It's convenient because it avoids the use of calculus in many common problems. It's convenient because it feels like a force if you're holding the end of a rope and spinning a bucket of water on the other end. It's convenient because in thinking of spacecraft in orbit, it's a force opposing that of gravity.

I understand your point: it's not really a force and from the viewpoint of a physicist, it's a cover-up of more fundamental concepts.

It's not my preference to replace "centrifugal force" with "intertia", but if that's a good way for you to think about it, that's fine. My likes run to thinking of such problems in terms of Newton's Laws of Motion (first and second) - if an object is following a curved path, then there's a component of acceleration directed toward the center of curvature and consequently there's a force proportional to the acceleration also directed toward the center of curvature.

 

[WarmWaterDiver]

Hopefully, Dr. Mike's in a better mood and had time to research the bouyancy of a lumped mass in air vs. vaccuo by now. These were principles explained during training for employment that didn't require an engineering degree, but financed my ability to study and complete a degree. The primary association involved with this training was API (American Petroleum Institute).

I studied Chemical Engineering for a number of years, when I could afford it, and have since practiced it for considerably more years than I studied it. I have no desire to spend more time in the world of pure academia - 10 years from start to finish of working and going to school off and on when I could afford it to get a bachelor's degree was enough for me. For force / vector resolution, I only dabbled as required to complete the program so I'll leave further work on that to Jonnythan and donacheson to discuss - but I'll have to say I've respected Jonnythan's material in other posts or threads I've read and been involved with.

 

[Bretagne]

Air pressure is a simple concept. Air pressure is the cumulative force exerted on a surface from the impact of gas molecules. It has nothing to do with the "weight" of the air column above your head as we are often taught as children (if it were the weight of the column of air, then pressurized cabins in space ships would be impossible - weightlessness). One of the consequences of the nature of gas pressure is that when you heat up the gas, the pressure increases (the heated up molecules move faster and thus strike surfaces with more force).

Water pressure seemingly has nothing little to do with the force from molecular impacts. Water at 90 degrees exerts the same pressure as water at 40 degrees (my guess is that this is due to the way a liquid heats with rotational and vibrational motions dominating over translation of the molecules). The weight of the water column doesn't quite seem to make sense either... wouldn't pressure from the weight of the water above you head push you down to the bottom - inside an underwater cave, wouldn't you need to take in account the weight of the rock over your head? Is it simply a matter of the water trying to occupy the space you are in? If that is the case, why does it increase with depth without depending on your volume (the amount of space the water is trying to reclaim)? What would be the pressure at the center of a sphere of water 66 feet across in a weightless environment?

Anyone have any insights into the nature of water pressure - what causes it?

In fact, you have it at the same time right and wrong.
Yes, pressure is due to molecular impact (in gas as well as in water). But these molecular impacts also have the effect to expand the fluid -> increase its volume. When does this expansion stops? when pressure strength is equilibrater by another strength. In an open environment (atmosphere, ocean), this equilibrium is reached when pressure strength is equal to the weight of the (water +) air column. When in confined area, by the resistance of the confinment material, eventually helped by outside pressure.

Which means that confinment is a way to keep pressure, if the confinment material (bottle or other) if completely rigid.
The proof that confinement keep pressure is the degaseing of the coca bottle when opened: before opening, high pressure -> no bubbles, after opening atmospheric pressure -> bubbles.
Now if the confinment material is only partly rigid, there will be a loss of pressure, but not a complete equalization with outside pressure.

Liquids (water) react differently than gas because the equation linking volume, temperature and pressure is no more PV=nRT, but it does not mean that there is no corresponding equation. Only liquids are almost incompressible, which means that volume changes, or temperature effect are very tiny, and neglected as a first approximation.