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Physics 8463 - 4.5 Forces

4.5.5 Pressure and pressure differences in fluids

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4.5.5.1 Pressure in a fluid

A fluid can be either a liquid or a gas.

The pressure in fluids causes a force normal (at right angles) to any surface.

The pressure at the surface of a fluid can be calculated using the equation:

pressure = force normal to a surface/area of that surface

p = F/A

This equation is NOT given on the Physics equation sheet - you need to know it, what the letters mean and are measured in!

p = pressure, in pascals, Pa

F = force, in newtons, N

A = area, in metres squared, m2

 

The pressure due to a column of liquid can be calculated using the equation:

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

p = hρg

This equation is given on the Physics equation sheet - but you need to know what the letters mean and are measured in - so I suggest learn it anyway!

p = pressure, in pascals, Pa

h = height of the column, in metres, m

ρ = density on fluid, in kilograms per metre cubed, kg/m3

g = gravitational field strength, in newtons per kilogram, N/kg

In any calculation the value of the gravitational field strength (g) will be given.

You should be able to explain why, in a liquid, pressure at a point increases with the height of the column of liquid above that point and with the density of the liquid.

You should be able to calculate the differences in pressure at different depths in a liquid.

A partially (or totally) submerged object experiences a greater pressure on its bottom surface than on its top surface.

This creates a resultant force upwards.

This force is called the upthrust.

You should be able to describe the factors which influence floating and sinking.
4.5.5.2 Atmospheric pressure

The atmosphere is a thin layer (relative to the size of the Earth) of air round the Earth.

The atmosphere gets less dense with increasing altitude.

Air molecules colliding with a surface create atmospheric pressure.

The number of air molecules (and so the weight of air) above a surface decreases as the height of the surface above ground level increases.

So as height increases there is always less air above a surface than there is at a lower height.

So atmospheric pressure decreases with an increase in height.

You should be able to:

describe a simple model of the Earth's atmosphere and of atmospheric pressure

explain why atmospheric pressure varies with height above a surface.