i am at a stage where i really need to have an in depth intuition about properties governing nature; and when i look at force, i can picture its effects it in my mind(change in movement,deformation) by replacing objects with force vectors and theoretically analyse them, but when it comes to pressure, i understand the definition but can not get an intuitive knowledge about pressure,especially in fluids. so i came up with my own theoretical experiments which might help me get an intuition about it, and i need a second opinion on the possible answers or further methods which might help me get a more comprehensive understanding of pressure.So,here are the questions:-

  1. Assume there is a $1m$ by $1m$ square flat plate. And I divide the area into $50$ by $50$ equal partitions. Then I apply point forces on every midpoint of the partitions all pointing downwards perpendicularly with magnitude $1N$, what is the pressure exerted on this $1m$ by $1m$ square flat plate? TRIAL: The answer is: $\frac{50 \times50\times1N}{1m\times1m}=2500Pa$.Initially i was tempted to say $1Pa$,but i later understood the definition of pressure and why it is a scalar quantity because it describes the SUM of all forces(perpendicular ones) present in a unit area.

  2. I have taken strength of materials course and worked with stress and strain.There is a concept about yield or ultimate strength which i totally understand, and there are corresponding stess values(yield or ultimate strength) to materials, for example, $A36$ steel has yield strength of $250Mpa$.when i say this statement, am i including atmospheric pressure? in other words, if i measure the yield strength in space, where there is negligible pressure is the yield strength of $A36$ steel a different value than the one i measured here in earth,maybe($250.1Mpa$)?

  3. While i am sitting down, I looked at the palm of my hand. and i measured a reasonable $4cm\times4cm$ square and i said to my self that around this sqaure area there is an air pressure of $0.1Mpa$,so a total force of $0.1M\times0.04\times0.04=160N$.To make things simpler i assumed gravity is $10\frac{m}{s^{2}}$.so it seems i am always holding a $16kg$ object.And if i am lying flat down and simply take a $15cm\times15cm$ square, a $225kg$ object is crushing my gut, so if i go to a theoretical planet with no atmosphere and gravity of $10\frac{m}{s^{2}}$, will my body withstand if a $225kg$ object is sitting on my body?

  • $\begingroup$ For item 3, if, on the theoretical planet, you would need to apply a normal pressure of 0.1 MPa over your whole body if you wanted to simulate the effect of earth's atmosphere. If you only did it over a 15x15 square, you skin would be bulging out over the rest of your body. And there would be a high stress on the edges of the square. $\endgroup$ – Chet Miller Oct 26 '17 at 16:21

1) You are right. You can even confirm this my measuring pressure exerted on each of those partitions. 1/50th of a meter is 2 cm. $2*2=4cm^2$.

$1N/4cm^2=2500 Pa$

2) I was studying material science and noone takes into account atmospheric pressure when talking about yield etc.

3) You need to remember that your body also have some pressure acting outwards, so it cancels out with pressure of atmosphere. If you go to a planet without atmosphere, your body still have some natural internal pressure. You will not explode, because your skin is rather solid balloon, but if something heavy sits on you - the internal pressure is not enough to counter the external pressure of thing that sits on you.

  • $\begingroup$ on question 3) since the 225 kg object is an approximate of the atmospheric pressure, why wouldn't the internal pressure be enough to resist the object? $\endgroup$ – Socre Oct 21 '17 at 16:58
  • $\begingroup$ on question 3) since the 225 kg object is an approximate of the atmospheric pressure, why wouldn't the internal pressure be enough to resist the object? $\endgroup$ – Socre Oct 21 '17 at 16:59

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