# Pressure in fluids

I've read (and experienced!) that pressure in fluids acts in all directions. Why is it so? I understand that if you dive 5 meters down in the water, the water's weight is greater than 1 meter below water surface. So if I'm standing up there under the water, I should feel a greater pressure on the top of my head. But why also on the side of my head?

Because the water molecules are bashing into you from all directions.

The large amount of water above you would be inflicting pressure on all of the water molecules below it, so these molecules are under even more pressure, and would 'want' to relieve that pressure by spreading out away from other molecules, just as if you squash a sponge, it spreads out and gets wider.

I'm not sure whether I've explained that well enough.

Pressure is an interesting topic. It is defined as a normal force force exerted by a fluid per unit area. If you were to draw a free body diagram for the object under consideration, you would have 4 normal forces perpendicular to your object (let's say a box). Thus, you would have an arrow going in on the left, bottom, right, and top, along with the weight of the object. You might know from physics that if an object exerts a force, that means there is an equal but opposite force also present. Pressure in a fluid increases with depth because more fluid "rests" on deeper layers, and the effect of this "extra weight" on a deeper layer is balanced by an increase in pressure. If you were calculating the pressure at a given point 10 meters below the surface of water, you would have to take P(atm) + $$\rho$$gz, $$\rho$$ being the density of water, and taking g to be 9.81 m/s$$^{}2$$. It's interesting to note that moving left and right in the water does not cause a pressure change.

Water is fluid, i.e even though the pressure of g comes from the top, it gets distributed cause of the fluid nature (and so the tendency to attain even pressure density).

I don't know if you can just say, "it gets distributed cause of the fluid nature." Of course it gets distributed because of its fluid nature. Also, I don't know what you mean by "even though the pressure of g comes from the top." When looking at pressure you have to understand that it is the normal force exerted by the fluid per unit area. You have to take into consideration the gravity involved at that point, along with the mass times the gravity of the object you're looking at -- perhaps a person underwater.

tiny-tim
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pressure is isotropic because …

I've read (and experienced!) that pressure in fluids acts in all directions. Why is it so? I understand that if you dive 5 meters down in the water, the water's weight is greater than 1 meter below water surface. So if I'm standing up there under the water, I should feel a greater pressure on the top of my head. But why also on the side of my head?

Hi Mårten!