# Pressure in Fluids,in particular horizontal pressure

I am not sure about the pressure existing in fluids.If the vertical pressure acts due to gravity and the atmospheric pressure since it also changes due to height also acts due to gravity then why is there horizontal pressure in a liquid. If there is then is it also equal to the vertical pressure at the point?

If gravity is suddenly stopped then do we still have water pressure and atmospheric pressure? In air is pressure equal in all directions? An interesting theory one of my friends explained is that if there is a box of air in space with no gravity and they have a velocity in one direction and due to elastic collisions there is pressure in all directions. But is it so that pressure in liquids act only due to moving molecules colliding randomly if there is no gravity?

Any explanations would be appreciated.

The up-down pressure from gravity/air meets the down-up resistance of container's bottom (and fluids are hardly compressible). This makes particles "look for their way out of the trap". The only option (if possible) is go sideways.

(If you exert additional pressure on the block of butter with the palm of you hand, it will get squeezed out sideways.)

Without gravity there is obviously no atmospheric pressure as well. It is gravity that retains air at the Earth's surface, so atmospheric pressure is the result of air's weight. Without gravity the atmosphere would have escaped into space.

Simple velocity (without acceleration) of a box in space changes nothing. Constant velocity means no force acting on the fluid inside the box (Newton's first law), so the fluid will exert equal pressure on all the walls of the box. (Gravity-generated pressure results from acceleration and not from gravity.)

EDIT: Gas or liquid is not held together like a solid body is. Their intermolecular attractions are weak compared to the kinetic energy of their molecules. These molecules propelled by their kinetic energy exert uniform pressure on all of the walls of the container it is held in (fluid would not normally exert pressure on the top wall of the container) as its molecules move around chaotically. If there is no gravity present, the value of this pressure depends on its kinetic energy, which varies with the temperature (compression is also a factor here).

Now, gravity is acceleration ($g=9,81 m/s^2$) and that's why it pulls air in the direction of the gravity source. If a gas would be held in a container placed in space without gravity, the pressure of the molecules would exert equal pressure on all the walls (that's the case I described in the paragraph above). Adding simple constant velocity would not change that (Newton: "When viewed in an inertial reference frame [i.e. at rest or with constant velocity], an object either remains at rest or continues to move at a constant velocity, unless acted upon by an external force [which is the source of acceleration].")

Going back to your first question to sum up. Horizontal pressure in liquid results from two factors - the kinetic energy of molecules (which makes them move chaotically in all directions) and the Earth's gravity (as molecules are pulled down by gravity and look for escape sideways).

• Thanks for taking the time to answer.Could you please explain the last paragraph in a little more detail?And also without gravity there is no air pressure?Won't air molecules still be moving around? – user45388 Apr 27 '14 at 21:52
• So if at a particular height the absolute pressure was P then if you place a sheet in the water what is the magnitude of horizontal force on the two sides of the sheet? – user45388 Apr 28 '14 at 7:50
• This is actually a new question. You should simply ask it in a separate thread, I think. – bright magus Apr 28 '14 at 7:56
• NOTICE, you received a down-vote from somebody for your question. It wasn't me, and I can only guess that this person thought you hadn't explored the subject on your own before asking. I suggest you take a look in Wikipedia or other internet sources for atmospheric pressure, states of matter, etc. And if you still can't understand something, then ask specific questions. This place is not considered an alternative to Wikipedia, but rather a way of assistance if regular sources are not clear enough to you (if you believe I answered your initial question then mark it as answered) – bright magus Apr 28 '14 at 8:01
• I never measured it. (I don't think I can add anything else meaningful to explain. I tried my best. If what I said is not enough for you, then you should wait for somebody else to answer it better.) – bright magus Apr 28 '14 at 18:16