4 ms·

why is it counter intuitive for you? It is not to me at all. Gravity pulls down. There is essentially no lateral component to gravity. Height is measured in the

why is it counter intuitive for you? It is not to me at all. Gravity pulls down. There is essentially no lateral component to gravity. Height is measured in the verticle dimension, the same as gravity. Now imagine water column as a stack of pennies. The more pennies are added to the stack the more pressure is on the lower pennies. It does not matter how many stacks are in front of or behind or to side of the stack you are looking at.

jvanderbot 5y ago
This is a great explanation, actually.
- JKCalhoun 5y ago
  Hmmm ... reinforces my counter-intuition. The stack of pennies might explain why the bottom of the jar would explode, but not the sides, area not below the stack of pennies. My intuition (wrong here) is that the extra surface not beneath the stack of pennies (your analogy) would in fact distribute the pressure and therefore represent a lower PSI on all sides of the jar.
  - jvanderbot 5y ago
    I guess I translated "pennies" into "little bags of water". The little bag of water at the bottom only gets pressurized from the little bags of water above it. And so the bottom bag's "pushing" outward from compression would be affected only by those above it.
Nathanael_M 5y ago
Super conter-intuitive for me as well. I appreciate your explanation!
RealityVoid 5y ago
Actually, yours is not such a great explanation, since a stack of quarters would manifest greater pressure than a stack of pennies. Whereas, a fluid column would manifest the same pressure no matter the diameter.
- Ma8ee 5y ago
  No, a stack of pennies would manifest a greater force, but the pressure would be the same. Pressure is force per area, which means that the increased weight from a wider column is exactly cancelled by the increased area that the force is distributed over.
  - Denvercoder9 5y ago
    Technically a stack of quarters would exert a greater pressure, but only because they're made of a denser material than pennies.
  - RealityVoid 5y ago
    Ok, yes, this holds true if the support surface increases the same. But the parallels still do not hold too well. Imagine sort of a funnel holding water, no matter the thickness of the base or top, the fluid pressure at the base is the same. Whereas with coins it does not work the same.
- morpheos137 5y ago
  penny is symbolic. diameter is irrelavent if scale is undefined.
Iv 5y ago
> There is essentially no lateral component to gravity Yet you can distribute the weight of a structure on larger bases. If you put two columns on pennies on a steel plate, the pressure under the plate will be higher than if you put only one column. It is easy to imagine the bottom vessel as similar to such a plate. I think the key to make this intuitive is to realize a few things: - pressure in liquids are transmitted differently - water is actually very slightly compressible - atmospheric pressure is also an important part of the system
- morpheos137 5y ago
  pressure is measured in force per surface area. It is disheartening to see HN so quantitatively illiterate.
  - simonh 5y ago
    Nobody is saying this fact about liquids isn’t true. We all know it’s true, so proving it doesn’t change anything. It is counter intuitive though precisely because the pennies analogy doesn’t work. If I put a 1m stack of quarters on a pressure gauge, then I put one quarter on the gauge and a stack of pennies on top of it up to 1m high, I get two different readings. Conversely if I measure the pressure at the bottom of two bottles the same shape as the stacks of coins, I get the same readings.
    - morpheos137 5y ago
      If you measure the pressure at the bottom of two bottles containing different density liquids the same shape as the two stacks of different coins you get two different pressure readings. In case it was not clear in my analogy a "penny" (it can be anything solid and incompressible) is representative of a water molecule. If you want to compare the pressures of stacks of pennies and quarters because for some reason you are fixated in these specific physical coins rather than what they represent then I will make it explicit. Imagine pennies represent water and quarters represent mercury. Two different liquids, two different coins, two different pressures. I don't get what you guys don't get. Can you explain to me why you think it is intuitive that the downward pressure of a liquid at the bottom of a container should vary for equal depths but different volumes? E.g. you seem to intuitively think that if you have a big pan filled with 1 inch of water the water pressure at the bottom of the pan is greater than the pressure at the bottom of a 50 ml flask also filled with one inch of water. This is false but I am interested as to why you and the others think it is "intuitive?" Where does the additional force come from to increase the pressure in the bigger pan? Gravity pulls straight down.

jvanderbot 5y ago

This is a great explanation, actually.

JKCalhoun 5y ago
Hmmm ... reinforces my counter-intuition. The stack of pennies might explain why the bottom of the jar would explode, but not the sides, area not below the stack of pennies. My intuition (wrong here) is that the extra surface not beneath the stack of pennies (your analogy) would in fact distribute the pressure and therefore represent a lower PSI on all sides of the jar.
- jvanderbot 5y ago
  I guess I translated "pennies" into "little bags of water". The little bag of water at the bottom only gets pressurized from the little bags of water above it. And so the bottom bag's "pushing" outward from compression would be affected only by those above it.

Nathanael_M 5y ago

Super conter-intuitive for me as well. I appreciate your explanation!

RealityVoid 5y ago

Actually, yours is not such a great explanation, since a stack of quarters would manifest greater pressure than a stack of pennies. Whereas, a fluid column would manifest the same pressure no matter the diameter.

Ma8ee 5y ago
No, a stack of pennies would manifest a greater force, but the pressure would be the same. Pressure is force per area, which means that the increased weight from a wider column is exactly cancelled by the increased area that the force is distributed over.
- Denvercoder9 5y ago
  Technically a stack of quarters would exert a greater pressure, but only because they're made of a denser material than pennies.
- RealityVoid 5y ago
  Ok, yes, this holds true if the support surface increases the same. But the parallels still do not hold too well. Imagine sort of a funnel holding water, no matter the thickness of the base or top, the fluid pressure at the base is the same. Whereas with coins it does not work the same.
morpheos137 5y ago
penny is symbolic. diameter is irrelavent if scale is undefined.

Iv 5y ago

> There is essentially no lateral component to gravity Yet you can distribute the weight of a structure on larger bases. If you put two columns on pennies on a steel plate, the pressure under the plate will be higher than if you put only one column. It is easy to imagine the bottom vessel as similar to such a plate. I think the key to make this intuitive is to realize a few things: - pressure in liquids are transmitted differently - water is actually very slightly compressible - atmospheric pressure is also an important part of the system

morpheos137 5y ago
pressure is measured in force per surface area. It is disheartening to see HN so quantitatively illiterate.
- simonh 5y ago
  Nobody is saying this fact about liquids isn’t true. We all know it’s true, so proving it doesn’t change anything. It is counter intuitive though precisely because the pennies analogy doesn’t work. If I put a 1m stack of quarters on a pressure gauge, then I put one quarter on the gauge and a stack of pennies on top of it up to 1m high, I get two different readings. Conversely if I measure the pressure at the bottom of two bottles the same shape as the stacks of coins, I get the same readings.
  - morpheos137 5y ago
    If you measure the pressure at the bottom of two bottles containing different density liquids the same shape as the two stacks of different coins you get two different pressure readings. In case it was not clear in my analogy a "penny" (it can be anything solid and incompressible) is representative of a water molecule. If you want to compare the pressures of stacks of pennies and quarters because for some reason you are fixated in these specific physical coins rather than what they represent then I will make it explicit. Imagine pennies represent water and quarters represent mercury. Two different liquids, two different coins, two different pressures. I don't get what you guys don't get. Can you explain to me why you think it is intuitive that the downward pressure of a liquid at the bottom of a container should vary for equal depths but different volumes? E.g. you seem to intuitively think that if you have a big pan filled with 1 inch of water the water pressure at the bottom of the pan is greater than the pressure at the bottom of a 50 ml flask also filled with one inch of water. This is false but I am interested as to why you and the others think it is "intuitive?" Where does the additional force come from to increase the pressure in the bigger pan? Gravity pulls straight down.