I don't really understand fluid pressure. From what I've read:

  1. Pressure is caused due to the weight of the fluid column
  2. But it acts in all directions equally at a given point because of the random movement of water particles

So why is pressure only dependant on gravity when it is also caused by the random movement of particles?

Also, even if you consider a water column, aren't particles entering and exiting the water column randomly? That means that no force is holding any particle or group of particles together or suspending them. So what really is pressure and how is it acting?

In zero gravity don't the particles still have random movement? Also since the acceleration due to gravity is downwards shouldn't any forces/pressures also be downwards?

Also I'm just a student so if possible please refrain from complicated terms. I'd like to get an intuitive understanding of fluid pressure in general. There are similar questions on the site but I'm not able to understand any of them.


1 Answer 1


You are right, that particle might move in and out of the fluid column randomly and that forces are exerted from particles within this column in all directions.

But you word randomly is important here. With so many particles moving randomly about, then there will statistically be just as many moving leftwards as rightwards at any given moment. Or in regards to forces and pressure, just as must force is exerted rightwards and leftwards (as well as backwards vs. forwards). All this cancel out when summed up.

Only the downwards force does not cancel out because is due to an external influence which causes a coherent downwards push with not-as-large an upwards push. That external influence is gravity.

  • $\begingroup$ But then shouldn't the net force/pressure in the sideways direction be zero? $\endgroup$ Nov 1, 2022 at 13:26
  • $\begingroup$ And why is the sidewards pressure also dgh? What relation does the random motion of the particles have with weight? $\endgroup$ Nov 1, 2022 at 13:34
  • $\begingroup$ @MarcCarlsan "But then shouldn't the net force/pressure in the sideways direction be zero?" Yes, exactly. $\endgroup$
    – Steeven
    Nov 1, 2022 at 16:42
  • $\begingroup$ @MarcCarlsan "And why is the sidewards pressure also dgh?" There is a pressure sideways. The point is that there is this same pressure in all directions. $\endgroup$
    – Steeven
    Nov 1, 2022 at 16:44

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