Pressure is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled gage pressure) is the pressure relative to the ambient pressure. Most of us perceive pressure by feeling or seeing its effects, such as when air blows on our skin or when we feel a firm handshake. Pressure also has a major effect on how fluids flow and interact with solids. All real-world objects are under some amount of stress from the forces around them; even objects like glassware and coins can be deformed at very high pressures.
The SI unit for measuring pressure is the pascal (Pa), which equates to one newton per square meter (N/m2). But in many situations, other units are more convenient. For example, atmospheric air pressure is often measured in millibars (mbar) or inches of mercury (inHg or ″Hg). Water depth can be measured in meters but it is common to use feet or fathoms. Blood pressures are stated in millimeters of mercury (mmHg). The United States customary units forpressure are pounds per square inch (psi) and pounds per square foot (psf).
Pressure may be represented by any of several types of gauges calibrated against each other: absolute, differential, gauge, and vacuum. Vacuum gauges measure pressures lower than surrounding atmospheric pressure—that is, they measure “negative” pressures or suction. Differential gauges compare two different readings; for instance two differentially-connected manometers may indicate how much higher one side must be raised to equalize fluid levels on both sides. Absolute gauges measure actual physical quantities such as mass flow rate through a venturi tube using an annunciator scale calibrated against standard gravity acceleration (~32 ft/s^2).. A tire gauge measures local air density and thus calculates instantaneous tire inflationpressure irrespective of atmospheric conditions. A barometer generates a reading by measuring hydrostatic head—the height difference between a columnof liquid mercury and the surrounding atmosphere—and thus indirectly indicating atmosphericpressure.:81–82:262 In medicine where low accuracies suffice, simple aneroid sphygmomanometers are usedto estimate arterial blood pressures by cuff deflationary methods,:182 but electronic oscillometricmethods have largely replaced them for more accurate readings at higher accuracies.:462–463
Atmospheric pressure decreases with altitude because there are fewer molecules above you pushing down from above. This relationship between altitude and atmospheric pressure was first discovered experimentally by Evangelista Torricelli in 1643,:38 who found that empty space had no weight and could hold only a limited amount of matter before becoming compressed like a solid body. From this he deduced that atmospheric pressuredecreases with height—a fact that had been previously observed but not explained.
The decrease in air density with increasing altitude causes sound waves to travel farther and faster through less dense media;:342 this explains why we can hear sounds made at great distances away better when we are high up, such as thunder while standing on top of a mountain rather than at its base. Conversely, sound waves travel more slowlyand compressively through denser media;(p37) this explains why sonar works best underwater where sound travels four times faster than in air.(pp49–50) As compressional waves, sound waves transfer energy through vibrational motion; however their speed limit depends on medium density so they cannot carry infinite amounts thereof like electromagnetic radiation can.
The speed limit c for all wave motion in any mediumis given by Newton’s wave equation: . Here ν=c/λis the phase velocity (speed at which individual wavefronts advance), ρis the medium’s mass density,,and Mis the medium’s shear modulus(a measure of stiffness). If either ρor Mincrease then cdecreases according to this equation—thus sound travels more slowlythrough denser materials like water than less dense gases like air despite having identical elastic properties because water has 1000 times greater mass density thanair.”