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Pressure, density & mass

"It's not about how bad you want it, it's about how hard you are willing to work for it." - an advice for your exams

Before, you get know what is density and pressure. We need to know what is mass

The formal definition of mass is the amount of inertia or resistant to change in motion. A less informal definition is the amount of matter or substance an object has


The mass per unit volume

SI units: kgm-3

Density = Mass/Volume

D = m/v

Sometimes they give density in gcm-3 So remember this:

1gcm-3 = 1000kgm-3


It is the force acting on per unit area

SI units: Pascals or Nm-2

Remember that 1 atm = 105Pa

Pressure in Liquids

Pressure in liquids act in all directions and it increases with depth and doesn't depend on the shape of the container

Pressureliquid = Density * g * depth

P = dgh

The derivation is sometimes asked in Mcqs

The derivation of liquid pressure in fluids in MCQ

P = hdg

So where is pressure examples used in:

  • Boyles law - A2
  • This is covered in A levels

  • Manometer
  • So this actually measures the pressure difference. The difference in height tells us the difference in pressure in term of hdg. In other words, the pressure at one end is greater than the other end by hdg

    Cambridge alevel physics revision notes- this diagram shows the derivation of a manometer
    So that extra pressure is shown by the liquid column. In order words, both sides must have the same pressure. So the liquid pressure caused by the liquid column and the atmospheric pressure is equal to the pressure of the gas

    So just remember this formula:

    Pgas= hdg + atm

    This only applies if the pressure of the gas is larger than atmospheric pressure

    These questions are usually confusing so watch this example to understand this fully

    Usually when the pressure difference halves the difference in height also halves. So on one side, the level decreases by 1/4 only.

  • Barometer
  • Similar to a manometer but, it is used to measure the atmospheric pressure. The height of mercury liquid provides a pressure which counteracts the pressure exerted by the atmosphere. So when the atmospheric pressure is equal to the mercury column pressure, we can find the pressure by using hdg to find the liquid pressure exerted by the mercury column

    Patm = Density of Mercury * Height * g

    It is useful to remember that the density of mercury is around 13600kgm-3


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