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What does pressure mean in science?
What is a unit of pressure?
Is pressure a force?
Apr 2, 2024 · pressure, in the physical sciences, the perpendicular force per unit area, or the stress at a point within a confined fluid. The pressure exerted on a floor by a 42-pound box the bottom of which has an area of 84 square inches is equal to the force divided by the area over which it is exerted; i.e., it is one-half pound per square inch.
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- Fluid Pressure
- Overview
- What does pressure mean?
- How do you find the pressure in a fluid?
- What's the difference between absolute pressure and gauge pressure?
- What's confusing about pressure?
- Example 1: Finding the pressure from the feet of a chair
- Example 2: Force on a submarine porthole
Pressure is kind of like force, but not quite.
What does pressure mean?
If you tried to hammer a bowling pin into the wall, nothing would probably happen except for people deciding to no longer lend you their bowling pins. However, if you hammer with the same force on a nail, the nail would be a lot more likely to penetrate the wall. This shows that sometimes just knowing the magnitude of the force isn't enough: you also have to know how that force is distributed on the surface of impact. For the nail, all the force between the wall and the nail was concentrated into the very small area on the sharp tip of the nail. However, for the bowling pin the area touching the wall was much larger, and therefore the force was much less concentrated.
To make this concept precise, we use the idea of pressure. Pressure is defined to be the amount of force exerted per area.
P=FA
So to create a large amount of pressure, you can either exert a large force or exert a force over a small area (or do both). In other words, you might be safe lying on a bed of nails if the total surface area of all the nail tips together is large enough.
If you tried to hammer a bowling pin into the wall, nothing would probably happen except for people deciding to no longer lend you their bowling pins. However, if you hammer with the same force on a nail, the nail would be a lot more likely to penetrate the wall. This shows that sometimes just knowing the magnitude of the force isn't enough: you also have to know how that force is distributed on the surface of impact. For the nail, all the force between the wall and the nail was concentrated into the very small area on the sharp tip of the nail. However, for the bowling pin the area touching the wall was much larger, and therefore the force was much less concentrated.
To make this concept precise, we use the idea of pressure. Pressure is defined to be the amount of force exerted per area.
P=FA
So to create a large amount of pressure, you can either exert a large force or exert a force over a small area (or do both). In other words, you might be safe lying on a bed of nails if the total surface area of all the nail tips together is large enough.
[Really?]
This definition also means that the units of pressure are newtons per square meter Nm2 which are also called pascals or abbreviated as Pa .
A solid surface can exert pressure, but fluids (i.e. liquids or gases) can also exert pressure. This might seem strange if you think about it because it's hard to imagine hammering in a nail with liquid. To make sense of this, imagine being submerged to some depth in water. The water above you would be pushing down on you because of the force of gravity and would therefore be exerting pressure on you. If you go deeper, there will be more water above you, so the weight and pressure from the water would increase too.
Not only can the weight of liquids exert pressure, but the weight of gases can as well. For instance, the weight of the air in our atmosphere is substantial and we're almost always at the bottom of it. The pressure exerted on your body by the weight of the atmosphere is surprisingly large. The reason you don't notice it is because the atmospheric pressure is always there. We only notice a change in pressure above or below normal atmospheric pressure (like when we fly in an airplane or go underwater in a pool). We aren't harmed by the large atmospheric pressure because our body is able to exert a force outward to balance the air pressure inward. But this means that if you were to be thrown into the vacuum of outer space by space pirates, your body pressure would continue pushing out with a large force, yet no air would be pushing in.
[So would you blow up?]
Okay, so the weight of a fluid can exert pressure on objects submerged in it, but how can we determine exactly how much pressure a fluid will exert? Consider a can of beans that got dropped in a pool as seen in the following diagram.
[Who dropped the beans?]
The weight of the column of water above the can of beans is creating pressure at the top of the can. To figure out an expression for the pressure we'll start with the definition of pressure.
When measuring pressure, people often don't want to know the total pressure (which includes atmospheric pressure). People typically want to know the difference in some pressure from atmospheric pressure. The reason is that atmospheric pressure doesn't change much and it's almost always present. So including it in your measurements can feel a bit pointless at times. In other words, knowing that the air inside of your flat tire is at an absolute pressure of 1.01×105Pa isn't really all that useful (since being at atmospheric pressure means your tire's flat). The extra pressure in the tire above atmospheric pressure is what will allow the tire to inflate and perform properly.
Because of this, most gauges and monitoring equipment use what is defined to be the gauge pressure Pgauge . Gauge pressure is the pressure measured relative to atmospheric pressure. Gauge pressure is positive for pressures above atmospheric pressure, zero at atmospheric pressure, and negative for pressures below atmospheric pressure.
The total pressure is commonly referred to as the absolute pressure Pabsolute . Absolute pressure measures the pressure relative to a complete vacuum. So absolute pressure is positive for all pressures above a complete vacuum, zero for a complete vacuum, and never negative.
This can all be summed up in the relationship between the absolute pressure Pabsolute , gauge pressure Pgauge , and atmospheric pressure Patm which looks like this,
Pabsolute=Pgauge+Patm
For the case of finding the pressure at a depth h in a non-moving liquid exposed to the air near the surface of the Earth, the gauge pressure and absolute pressure can found with,
People often want to plug in the density of the object submerged ρobject into the formula for gauge pressure within a fluid P=ρgh , but the density in this formula is specifically referring to the density of the fluid ρfluid causing the pressure.
People often mix up absolute pressure and gauge pressure. Remember that absolute pressure is the gauge pressure plus atmospheric pressure.
A 7.20 kg fuchsia colored four legged chair sits at rest on the floor. Each leg of the chair has a circular foot with a radius of 1.30cm . The well engineered design of the chair is such that the weight of the chair is equally distributed on the four feet.
Find the pressure in pascals between the feet of the chair and the floor.
P=FA(Use definition of pressure. Gauge pressure isn’t applicable here since there’s no fluid.)
P=mgA(Plug in formula for weight of the chair W=mg for the force F)
P=mg4×πr2(Plug in the total area of the feet of the chair 4×πr2 for the area A.)
P=(7.20 kg)(9.8ms2)4×π(0.013 m)2(Plug in numbers, making sure to convert from cm to m)
A curious seahorse is looking into the circular window of a submarine that is sitting at a depth of 63.0 m underneath the Mediterranean sea. The density of the seawater is 1025kgm3 . The window is circular with a radius of 5.60 cm . The seahorse is impressed that the window does not break from the pressure caused by the weight of the seawater.
What is the magnitude of the force exerted on the surface of the circular submarine window from the weight of the water?
P=FA(Use the definition of pressure to relate pressure to force)
F=PA(Solve the formula symbolically for the force)
F=(ρgh)A(Plug in the formula for gauge pressure Pgauge=ρgh for the pressure P)
F=(1025kgm3)(9.8ms2)(63.0 m)(π×[0.056 m]2)(Plug in numbers for ρ,g,h, and A)
Dec 28, 2020 · Pressure is a key concept in physics, whether you're interested in how meteorologists predict the weather, or you want to describe the Carnot cycle in thermodynamics. Pressure is the force on a surface per unit area, and it has a close link to the temperature of the liquid or gas creating it.
Pressure is defined as a measure of the force applied over a unit area. Pressure is often expressed in units of Pascals (Pa), newtons per square meter (N/m 2 or kg/m·s 2 ), or pounds per square inch. Other units include the atmosphere (atm), torr, bar, and meters sea water (msw).
Definition. Pressure is the amount of force applied perpendicular to the surface of an object per unit area. The symbol for it is "p" or P. The IUPAC recommendation for pressure is a lower-case p. However, upper-case P is widely used.
- p, P
- pascal [Pa]
May 7, 2019 · In science, pressure is a measurement of the force per unit area. The SI unit of pressure is the pascal (Pa), which is equivalent to N/m 2 (newtons per meter squared). Basic Example. If you had 1 newton (1 N) of force distributed over 1 square meter (1 m 2 ), then the result is 1 N/1 m 2 = 1 N/m 2 = 1 Pa.
Pressure is force per unit area of surface; the SI unit for pressure is the pascal (Pa), defined as 1 newton per square meter (N/m 2). The pressure exerted by an object is proportional to the force it exerts and inversely proportional to the area on which the force is exerted.
