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This is gauge pressure. Absolute pressure is technically what we think of when we say pressure - the force that the gas is applying per unit area of the container. If the gas is applying 101,300 Newtons per square meter, then the absolute pressure would be 101.3 kPa.
It is possible for the absolute pressure to be negative. When this occurs, we say the water is under tension. If there are weaknesses in the water (air bubbles, interfaces, etc.) then the water will cavitate and you will end up with the situation described by @Alan Rominger.
Of course, you can easily have a $20\: \mathrm{PSI}$ pressure differential (although not without pressure above $1\: \mathrm{ATM}$ since that's $14.22\: \mathrm{PSI}$ at sea level). Check out Wikipedia on the zero-reference: Absolute pressure is zero-referenced against a perfect vacuum, so it is equal to gauge pressure plus atmospheric pressure.
Gauge pressure is zero-referenced against ambient air pressure, so it is equal to absolute pressure minus atmospheric pressure. Negative signs are usually omitted. Negative signs are usually omitted. To distinguish a negative pressure, the value may be appended with the word "vacuum" or the gauge may be labeled a "vacuum gauge."
$\begingroup$ @Ali_CHENG Yes, you can convert the numbers back to absolute scale, but why bother with the gauge pressure in the first place when you always need the absolute scale to determine phases. In thermodynamics, phases are what you are usually concerned with.
The gauge pressure will affect the shape of the tire without a load on it. The deformation of the tire once load is applied will depend on both the gauge and absolute pressure. And the distribution of force across the footprint will depend more heavily on the absolute pressure.
The intent is for you to use the product of Pressure and the total Area to solve for a force (weight) from which you can determine the mass of the car. I solved for the Absolute Pressure (including atmospheric pressure) before plugging it in to solve for force.
$\begingroup$ Absolute pressure is the actual pressure. At point D the absolute pressure is the absolute pressure at C, which is 0, plus the pressure due to the 0.5 m of Hg. Gauge pressure is by definition: pressure relative to the atmosphere = absolute pressure minus atmospheric pressure. The problem calculates absolute pressure at D (same ...
But negative (absolute) pressure has no physical meaning, so how can we know if the Bernoulli Equation application will lead to robust results? A negative pressure would represent an "exhaustion" of static pressure to convert in kinetic energy, so in these conditions the velocity field cannot be realized and the Bernoulli equation would be ...
In your calculation using absolute pressure, did you include the pressure force acting on the outside of the pipe bend? $\endgroup$ – Chet Miller Commented Feb 6, 2020 at 13:02