Pressure

Have you ever wondered why the sharp edge of a knife can cut through a fruit seamlessly whereas if you used the blunt edge it would require a lot more effort? This phenomenon can be explained by understanding the concept of pressure. In this case, information about the magnitude of the force that is exerted by the knife is not enough to know whether or not cutting fruit is easy or not. We need another measure that quantifies how much force is exerted over a particular area. At the end of this article, you will have learned what pressure is, learned how to use the pressure equation, and worked on a few examples to understand the importance of pressure in our daily lives.

Pressure Pressure

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Table of contents

    Pressure definition

    Before we introduce the formula for pressure and look at some worked examples, let us first define what we mean by pressure in physics.

    Pressure is defined as the force exerted per unit area of a surface.

    The amount of force exerted by a knife is not enough information to know whether or not we will be able to cut into a fruit. Pressure takes into account how concentrated the force is over a particular area.

    Pressure formula

    So how do we calculate the concentration of force in a given area? Simple, we know that pressure is nothing but the force acting per unit area which in mathematical terms give

    P=FA

    or in words

    id="2827891" role="math" Pressure=Force normal to the surface Area

    Pressure is expressed in Pascals (Pa),Fis the force inN, and the area is measured in units ofm2.

    Pressure The image shows the concentration of forces with respect to area StudySmarterThese knives have different contact areas with the surface, the sharper knife exerts more pressure for a given amount of force because the force is acting over a smaller surface area, StudySmarter Originals

    A pressure of1 Pacan be defined as a force of1Nacting on an area of1m2(unit area). The above formula is only valid when the force acts at right angles to the surface. As you can see from the equation. Pressure is directly proportional to the force applied and inversely proportional to the area on which it acts. This means that to increase the pressure we can either

    • Increase the force keeping the area constant or

    • Decrease the area keeping the force at a constant.

    Pressure Diagram comparing a hammer hitting on a basketball and a nail StudySmarterAs the area of contact decreases, the pressure increases provided the force remains the same, this allows the nail to be driven into the wall, StudySmarter Originals

    Units of pressure

    The SI unit of pressure isPa or N/mm2 1 Pascal = 1 N/mm2. For large quantities of pressure, we can use 1 kPa = 103 Paand1 MPa= 106 Pa. You may also see other units of pressure such as torr, barr, atm, psi and mm Hg.

    The atmospheric pressure is a commonly used unit (atm). The pressure exerted by the Earth's atmosphere on the surface of the earth is what is known as atmospheric pressure. The atmospheric pressure in pascals is equal to101325 Pa. It is also sometimes referred to as standard atmospheric pressure.

    Types of pressure

    We can categorise types of pressure in terms of the states of matter that are exerting the pressure. In this section we'll have a look at each of the types of pressure and some examples of each type too.

    Pressure can be exerted by solids, liquids, and gases. Solids exert pressure through their point of contact. Liquids and gases exert pressure on a solid due to the collision of their particles with the solid.

    • A nail being hammered into a wall is an example of pressure acting from one solid onto another solid.
    • We are constantly facing the force of the atmosphere on our bodies. which is nothing more than the pressure exerted by the gases in the atmosphere.

    The pressure exerted by liquids

    The pressure exerted by liquids is mainly due to the weight of the liquid. Imagine taking a dive at your local swimming pool. Let's say you dive into the deep end. Now due to the effects of gravity, all of the weight of the water above you will act to press the water molecules down onto your body and any other object submerged in the water. This pressing is what we mean by the pressure exerted due to a liquid. The number of water molecules that are present above you will increase as you keep going deeper. This is why the pressure exerted by liquids increases with increasing depth and it's why the pressure you feel increases as you dive deeper into a body of water. Another factor that comes into play is the density of the liquid. Since density measures the mass per unit volume of a liquid. Liquids with higher density will exert a larger pressure at the same depths due to their larger weights.

    Pressure A diagram showing pressure in liquids StudySmarterThe pressure exerted at a particular depth on a base Area A can be calculated by considering the weight of the column of liquid directly above A, StudySmarter Originals.

    Lets now consider how to calculate the pressure exerted by a liquid at a particular depth. We consider a rectangular column of water with a base areaAand heighth.

    We already know that the formula for pressure is:

    P=FA

    The weight of the liquid situated directly above the base area of considerationAis given by the following equation:

    F=mg

    Or in words

    Weight = mass × gravitational field strength

    The next step in our calculation was considered to consider the density of the liquid in order to calculate the mass of the column of water:

    ρ= mV,

    or in words,

    Density=massvolume.

    Rearranging for mass we get:

    m=ρV.

    The volume of a cuboid can be written as the product of its base areaAand its heighth:

    V=Ah,

    or in words,

    Volume =Base area×height.

    We Substitute the formula for weight in terms of mass before substituting the formula for mass in terms of density and volume. Finally, we substitute the formula for volume in terms of base area and height into the equation for pressure:

    P=mgA=ρ×V×gA=ρ×A×h×gA

    After further rearrangement, we arrive at the formula for the pressure exerted by a column of liquid pressure in terms of the density of the liquidρ, the height of the column of liquidh, and the base area of the column of liquidA:

    P=ρhg

    pressure = height of the column × density of the liquid × gravitational field strength

    Where pressure is measured in Pascals (Pa), the depth or height of columnhin meters (m) and densityρinkg/m3and gravitational field strength,ginN/kg.

    Next, let us look at a few examples where we use these formulas to calculate the pressure.

    Atmospheric pressure

    Atmospheric pressure is due to the air molecules in the atmosphere colliding with the earth or any other object contained within. The atmospheric pressure reduces as the altitude increases because the density of the air decreases at higher altitudes. At higher altitudes, there is less air, therefore the weight of the air pressing down on an object is reduced. This is the reason why some people experience ear pain during air travel that occurs due to quick changes in air pressure. The atmospheric pressure can be calculated using the same formula as the pressure due to liquids.

    Pressure A cube diagram indicating the atmospheric pressure StudySmarterAtmospheric pressure is due to the weight of the air molecules directly above the surface of the Earth, StudySmarter originals

    Examples of pressure

    A fluid has a weight of300 Nwhich is exerted over a surface area of8.2 m2. Calculate the pressure acting on the surface.

    F=300 N , A=8.2 m2P=300 N8.2 m2=36.585 Pa

    Calculate the pressure exerted on the ground by a person weighing60 kgwearing

    • A flat shoe with a contact area of100 cm2
    • A heeled shoe with a total contact area of26 cm2

    What can you infer from the results?

    Pressure A diagram that has a man shoe and heels side to side StudySmarterHeels vs Flat shoes, Nidhish Gokuldas StudySmarter Originals

    Unit conversion

    1 cm = 0.01 m 1 cm2 =0.0001 m2 100 cm2 = 0.01 m2 26 cm2 =0.0026 m2

    Pressure on the ground due to flat shoes

    m=60 kg , A=0.01 m2P=FAF=mg=60 kg × 9.8 m/s2=588 NP=FA=588 N0.01 m2=58,800 Pa = 58.8 kPa

    Pressure on the ground due to heels

    m=60 kg , A=0.0026 m2P=FAF=mg = 60 kg × 9.8 m/s2 = 588 NP=FA=588 N0.0026 m/s2=226153.846 Pa = 226.15 kPa

    The pressure exerted by the heels is almost 5 times that of the flat sole shoes. This is the reason why heeled shoes are uncomfortable to wear over long periods as they exert a large amount of pressure on the ground and equally on the persons who's wearing the shoe's feet.

    Calculate the pressure in the water well at a depth of50 m. Take the density of water to be1000 kg/m3.

    Step 1: List the given quantities

    ρ=1000 kg/m3, h=50 m, g=9.8 m/s2

    Step 2: Calculate the pressure using the appropriate equation

    P=ρhg

    P = 1000 kg/m3 × 50 m × 9.8m/s2 = 490,000 Pa = 490 kPa

    Calculate the density of air at an altitude of5 kmwhere the pressure of the surrounding atmosphere is12.5 kPa.

    Step 1: List the given quantities

    h=5 km =5000 m, g=9.8 m/s2, P=12.5 kPa

    Step 2: Calculate the pressure using the appropriate equation

    P=ρhgρ=Ph×gρ=12.5×1000 Pa5000 m×9.8 m/s2ρ=0.25 Kg/m3

    Pressure - Key takeaways

    • Pressure is defined as the force exerted per unit area of a surface.
    • Pressure can be calculated by dividing the force by the area across which it is acting:

      P=FA or in words, Pressure=Force normal to surfaceArea.

    • There are different types of pressure depending on the type of matter exerting the pressure.

    • The pressure exerted by a liquid is given by P=hρg.

    • The pressure exerted by a liquid increases as the depth of the liquid increases.

    • Gases exert pressure on the objects they come into contact with.

    • Atmospheric pressure is due to the weight of the air molecules on top of the earth.

    • Atmospheric pressure reduces as the altitude increases.

    Frequently Asked Questions about Pressure

    What is pressure?

    Pressure is defined as the force per unit area.

    How do you calculate pressure?

    The formula for pressure is P=F/A

    What is the formula for pressure?

    The formula for pressure is P=F/A

    What is an example of pressure?

    A knife cutting a slab of butter, A nail being hammered into a wall are examples of pressure

    What are the types of pressure?

    The types of pressure are 

    • Absolute pressure.
    • Gauge pressure.
    • Differential pressure.
    • Sealed pressure or vacuum pressure.

    Test your knowledge with multiple choice flashcards

    The unit for pressure ...

    What happens to pressure when the area increases?

    What happens to pressure when force increases?

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