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I am informed that the pressure is due to the collision of particles on the surface.If so on the surface of water in a glass, the air particles could collide to water surface in many different directions and not just normal to the surface.So how can we say pressure if Force normal to surface divided by area?.Even if it is a definintion of force.What is the parallel(horizontal component) doing to the water surface (definitely not moving water in hydrostatics).

  Am I missing something here?.
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  • $\begingroup$ Because the horizontal components (of the particles' motion) are equal and opposite, summing to zero, whereas the vertical components are all in the same direction? $\endgroup$
    – Weather Vane
    Jul 2, 2021 at 12:51
  • $\begingroup$ Your both points are correct. 1) collision part 2) Pressure definition. But I think you are not properly imagining it. Consider a Boat in water and think about the surface in contact of the boat and water $\endgroup$
    – gobbledy-gook
    Jul 2, 2021 at 13:00

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Weather Vane is basically correct. The atoms or molecules of a fluid are bouncing around at random. Assuming the surface being impacted is rough enough at the sub-microscopic to receive forces components which parallel to the surface, then these forces would be randomly distributed in magnitude and direction. The resultant force can only be normal to the surface.

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