Pressure: Rate or Distributed Force?

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Discussion Overview

The discussion centers around the concept of pressure and whether it can be classified as a rate. Participants explore the definitions of rate, the relationship between pressure and force, and the implications of these definitions in physics and everyday language.

Discussion Character

  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants propose that pressure is a measure of force distributed over an area, which does not fit the conventional definition of a rate that involves time.
  • Others argue that pressure can be considered a rate in a broader sense, as it can be expressed as a quantity per unit area, similar to other rates like "dollars per pound."
  • A participant mentions that rates are often defined in terms of time, suggesting that pressure does not change over time and thus should not be classified as a rate.
  • Some contributions highlight that various definitions of rate exist, including those that do not involve time, such as lapse rate and tax rate.
  • There is a suggestion that pressure can be integrated over a surface to yield total force, which could imply a rate-like behavior in certain contexts.
  • A later reply questions the utility of classifying pressure as a rate, noting that it is not commonly used in that manner in practical applications.
  • One participant raises a hypothetical question about the existence of pressure if time were "frozen," prompting further exploration of the concept.
  • Another participant connects pressure to concepts in general relativity, suggesting a deeper relationship between pressure and momentum flux.

Areas of Agreement / Disagreement

Participants express differing views on whether pressure should be classified as a rate. While some agree that pressure does not fit the conventional definition of a rate, others maintain that it can be viewed as a rate in certain contexts. The discussion remains unresolved with multiple competing perspectives.

Contextual Notes

Participants note that definitions of rate can vary significantly, and the discussion reflects a mix of practical and theoretical considerations regarding the classification of pressure. There are unresolved assumptions about the implications of defining pressure as a rate and its utility in various scientific contexts.

alkaspeltzar
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My question is pressure a rate? Rate is defined as a some measure per another measure or quantity. Doesn't pressure fit that definition? yet everything I read says no...pressure is not a rate? Same with density

Is it because pressure is really a distributed force per area, and it is not a value dependent upon another. Like mph(speed) is change in distance per time. Can someone please explain this aching thought in my head?

Sorry if this is incorrect forum, just looking for help.

Thankyou
 
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Rate is the change in any quantity with respect to time, and time only.
 
Okay, I agree with that and that's what I always thought.

But then there are definition and examples of rates like " $ per pound " so how does that work?
 
alkaspeltzar said:
Okay, I agree with that and that's what I always thought.

But then there are definition and examples of rates like " $ per pound " so how does that work?
Words in everyday language can be used very loosely and have found various applications, as is the case over here. In physics, however, word meanings are very specific and non-interchangeable.
 
PWiz said:
Rate is the change in any quantity with respect to time, and time only.

Well that's not true. A rate, as one definition in the dictionary puts it, is:

4a :a quantity, amount, or degree of something measured per unit of something else <her typing rate was 80 words per minute>

In that sense, a pressure would be a rate. It's the rate at which force on a surface under pressure changes per unit of area considered. It's a rather unconventional definition of rate, but it would still technically be a rate.

More importantly, why does it matter? It's just semantics and doesn't really affect the physical interpretation at all.
 
PWiz said:
Rate is the change in any quantity with respect to time, and time only.

Not so fast. What about:

lapse rate
literacy rate
those are at least somewhat "sciencey"

not to mention:
tax rate
exchange rate
interest rate

etc.
etc.

But I would really hesitate to call pressure a rate...

oops looks like boneh3ad beat me to it...
 
Actually... What ur all saying makes sense. From a practical physics or science background, rates always include time. Therefore pressure is not. Its not something changing or defined by a particular unit of time.
But the term does get thrown round loosely, and therefore we see things like percentages called rates or things like exchange rates.

I will not worry about it. Pressure is pressure, its force applied over area, as in psi. Rates are quantities depending on time, like speed and flow rate. That's good enough.

PS. English fudges word meaning all the time
 
alkaspeltzar said:
Actually... What ur all saying makes sense. From a practical physics or science background, rates always include time. Therefore pressure is not. Its not something changing or defined by a particular unit of time.
But the term does get thrown round loosely, and therefore we see things like percentages called rates or things like exchange rates.

Actually that's not what I am saying at all. There are many practically important rates in mathematics and physics that do not involve time: rate of change of a quantity as a function of space, angular frequency versus wavenumber, pressure versus density, and many others.

Pressure just isn't very useful as a rate because it doesn't really pop up anywhere useful in that fashion. It's still a rate, though, in the sense that you can integrate it over a surface exposed to variable pressure (such as a vertical wall in a water tank) in order to get the total force on that surface. You don't really ever see it done in the opposite direction, though, as pressure is the more fundamental property.
 
Okay..I guess now I am just confused. You say its a rate but then not useful as a rate. So is it or is it not?

To me it doesn't seem like one as it doesn't fit the description as compared to speed or acceleration or something that is one unit based on another, like time.
 
  • #10
Looking online...there are many definitions of rate. Some deal with money, many in sciences deal with time/frequency. For application i think I over confused that and started over analyzing pressure as one based on how we express it. I know what it is, I guess I am not going to concern myself with its strict definition. Thanks
 
  • #11
If you "froze" time would pressure still exist? I know you can measure pressure at any given time, but if you froze time would pressure still be exerted?
 
  • #12
boneh3ad said:
Pressure just isn't very useful as a rate because it doesn't really pop up anywhere useful in that fashion.

Am I missing something? We routinely calculate forces by multiplying pressure by area. Isn't that an example of using pressure as a rate?

If not what about pressure vs depth as in "0.1 bar per meter of water" ?
 
  • #13
In GR the energy-momentum tensor's (1,1), (2,2), (3,3) diagonal components are momentum flux or flow and are equivalent to pressure aren't they?
 

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