# If an unsealed water bottle has three holes at a different heights, why would the lower hole give out the most water? Is it because of air pressure?

If an unsealed water bottle has three holes at a different heights, why would the lower hole give out the most water? Is it because of air pressure? I am guessing it's because air pressure pushed on the water and the hole at the bottom gives out more water because the pressured water pushes on it. I am not sure, though. Can anybody help me before my test? I will really appreciated.

Thank you.

This has nothing to do with air pressure since the air pressure is almost exactly* the same at all of the holes (including the top one). Regarding pressure only differences can cause stuff to move, the absolute pressure is only relevant for density and such.

The concept you should read more about is called hydrostatic pressure and given by a very simple formula. The formula basically says that in an (incompressible) liquid (like water) the pressure is proportional to the depth you are in. Of course the strenght of gravity and the density of the fluid also plays a role, but in most situations this is water and 9.81m/s anyway.

If you look at this topic from the right angle it should become very easy; I'd therefore encourage you to think about it (based on your everyday knowledge/experiments) until it does.

* I oversimplified a bit: in fact the air pressure increases the lower you go, which slightly counteracts the effect you see in the experiment. Since air is much less dense than water however, this can be neglected in practice.

• "This has nothing to do with air pressure since the air pressure is exactly the same at all of the holes (including the top one)." This is patently untrue. The difference in pressure might be negligible compared to the difference in water pressure due to the water's weight, but there is a difference. This difference is how some altimeters work. – Shufflepants Feb 12 '19 at 19:41

There are two reasons why the bottom hole puts out more water.

1, it is exposed for a longer period of time than the two holes above it, add,

2, the velocity of the water coming from the lowest hole in greater than the other two holes above it. See formula below:

V = sq. root of 2gh where:

V = velocity in ft/s g = accel const. 32.2 ft/ s^2 h = height of liquid in feet.

Since the bottom hole has more h to the liquid surface, the V is always faster than the two holes above it.

gravity will push the water down. more water is on top of the water being pushed out. so the water will be pushed out hardes on the bottom will be pushed out.