Fluid Pressure

The pressure at a point in a gas or liquid which is at rest acts equally in all directions


Consider a submarine which is at rest at some point under sea at particular height.  Then the total amount of pressure acting on the submarine will always cancel each other out otherwise the submarine can never be stationary at one position.

Therefore, we can safely say that if you consider any point in gas or liquid which is at rest, then the pressure at that point from all directions stays the same; otherwise the gas or liquid would move in the direction of low pressure.

In above diagram a point object is shown and direction of pressure is shown with arrows. All arrows are of equal sizes which shows pressure is same from all sides.


pressure difference = height × density × g

p = h×ρ × g


This relationship establishes that pressure of fluids (liquids and gases) varies with depth and it increases as the distance (depth or height)  increases or decreases.

In this equation, density of water stays same which is 1000 kg/m3 and acceleration due to gravity also stays same as 10 m/s2; while height should be measured from the previous point to new location.

For example: if you are going further down the bottom of the sea you will experience more pressure. Water pressure varies enormously with depth as after every 10 m from the pressure increases to 100 Kpa or 1 atm. For solid objects like our bones and muscles, this added pressure is not much of a problem; but it is a problem for any air-filled spaces like the mouth, ears, paranasal sinuses and lungs. That’s why scuba divers find it difficult to go down more than 20 m without great difficulty.

Suppose you have a container of water of height 10 meter. You put a very thin object at a height of 2 meter then you would observe that the amount of pressure onto this object will remain the same at this height. Even if you move the object horizontally at the same height pressure stays the same.

Try the following example

An aquarium has a tunnel through a tank of water at a depth of 5 m below the surface. The manufacturer guarantees the tunnel to a pressure difference of 200 kPa. Is the tunnel safe?



Walls of the dam are thicker at the bottom because pressure increases with depth and may destroy the wall from bottom if maintained same thickness.



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