Thread: pressure

had some trouble figuring out how to calculate pressure with the unit pascal.

what i did was this.
kilos / m^2. is this wrong?

i fiddled around with google converting from PSI to pascals, and the answers only matched up approximately, in addition to the comma being shifted 1 place too many, e.g instead of getting 88 pascals, i got 8.76, and stuff like that with the above equation. so i dunno if theres something wrong with either my understanding of the physics, or google.

pressure makes picks baby

Look, what I've found.

http://en.wikipedia.org/wiki/Pascal_(unit)

Originally Posted by dejawolf

what i did was this.
kilos / m^2. is this wrong?

It’s newtons per m², not kilos per m².

Why would you want to bother with PSI? :? Just stick to the SI unit of Pascal.

It’s newtons per m2, not kilos per m2.

And that accounts for the decimal point discrepancy, the factor of g=9.81.

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Originally Posted by SteveF

It’s newtons per m2, not kilos per m2.

And that accounts for the decimal point discrepancy, the factor of g=9.81.

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That’s for pressure as weight per unit area. Pressure in general, as force per unit area, need not be related to g.

Agreed. But he started out with pounds, ended up with kilograms instead of newtons. There's the 9.81 conversion factor.

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i did not start out with pounds, i used them to check if i got things correct with the pascal. which i didn't.
i knew about the wiki page on pascals, but it only managed to confuse me.

i tried to stick to pascal, but i only managed to get it wrong, which is why i'm asking here on the forum what exactly i did wrong.
as for newton, i was trying to figure out the pressure of an object standing still on the ground. which means it wouldn't be accelerating at all.
so i figured, wouldn't that cancel out the acceleration part?

hmm, wait.

On Earth's surface, a mass of 1 kg exerts a force of approximately 9.81 N [down] (or 1 kgf). The approximation of 1 kg corresponding to 10 N is sometimes used as a rule of thumb in everyday life and in engineering.

its still confusing though. i know earths gravity pulls stuff down at a speed of 9.81m/s^2. basically an exponential growth in speed.

Originally Posted by dejawolf

i knew about the wiki page on pascals, but it only managed to confuse me.

1 pascal (Pa) ≡ 1 N·m−2 ≡ 1 J·m−3 ≡ 1 kg·m−1·s−2

Isn't that obvious? It tells you exactly, what 1 Pa is.

Originally Posted by dejawolf

On Earth's surface, a mass of 1 kg exerts a force of approximately 9.81 N [down] (or 1 kgf). The approximation of 1 kg corresponding to 10 N is sometimes used as a rule of thumb in everyday life and in engineering.

its still confusing though. i know earths gravity pulls stuff down at a speed of 9.81m/s^2. basically an exponential growth in speed.

I suppose the confusion is based on the common misinterpretation of the identity of mass and weight. In physics, there is a difference. If you put something in open space far away from a massive body, it still keeps its mass, but the weight it experiences is zero. It is Einstein's equivalence principle that says that it is impossible to distinguish between weight and acceleration. Hence the similarity in the earth's gravitational constant g.

In summary:

Every rigid body has a mass. If you let it fall to earth, it is accelerated with a rate of g=9.81 m/s^2 (mean value between the equator and the poles, and close to the earth's surface). It gains 9.81 m/s of velocity every second (however the friction of the earth's atmosphere limits the final speed). When it hits the surface, it cannot fall anymore, but instead it is pulled down with the same acceleration that now is experienced as weight, i.e. a force that depends proportionally on the acceleration and the mass of the body. Therefore, you need a quantity containing both to describe it. The unit is N (Newton) = kg*m/s^2. Then you can generalise it to any force, not only gravity. A force is the quantity that tells you how strong a momentum can affect other things. Pressure is the force per area unit, i.e for a pressure it does not matter, if you have 10 N per m^2 or 100 N per 10 m^2. The force per area unit is the same.