# Hydrostatic Pressure - the pressure of Fluids

Water is heavier than you think

You’ve probably noticed it when an object like a ball is at the surface of the pool, that it seems fine. But what if it were to go to the bottom of the pool? The ball ends up feeling a pressure exerted on it. Now if you were to go even deeper, like to the ocean, the ball would probably get crushed. Well, this is because of a concept known as hydrostatic pressure, which explains the pressure that a fluid exerts on an object. Now if you want to look at different physics concepts, you can look at our physics question review for light refraction. Pretty interesting stuff, but I digress.

Why does the ball get crushed?

Now, the pressure changes depending on how far down the object is submerged in water and this is due to the weight of water above the object. Think about it, if you were to start at the water surface, there would be no weight of water on the ball, but the further down you go, the more weight the water exerts onto it just because there’s more water that is above it. The equation that you will most likely see in a physics classroom that explains this is:

P=ρgh

Skipping all the derivations, this states that the pressure is equal to density of the fluid, gravity, and the height the object is submerged all multiplied with one another.

So is it like this for every situation?

Good question. Now this equation is assuming that the fluid is not exposed to any other conditions. In the real world, water that is exposed to the outside air also experiences pressure from the air known as atmospheric pressure. Same concept as water, air also exerts a weight onto objects though not as great as water, which makes sense because if it wasn’t the ball would be as flat as a pancake. So our previous equation can be written as:

P=ρgh+Patm

But in most cases when you encounter a physics problem, you’ll just assume that atmospheric pressure is negligible with a value of zero. This is a very fundamental concept in the realm of fluids and is applicable in the hydrostatics and hydrodynamics fields. Now, an interesting topic to cover is how this concept was derived. Food for thought.