# Water Bottle

## Main Question or Discussion Point

Today when I was toying with a water rocket I came upon a strange phenomena. The setup was like this:
I had a 625 mL coke bottle, filled half way with water. The cap had a nice hole in it, about 1.5 cm. I turned the bottle upside down, thinking that the water would just gush out, but low and behold it kept in there, only dripping once every 4 seconds or so. I tried this again, and the same result occurred
So heres my question, why did it do that? Why didnt the water just gush out at once (like it should with the force of gravity acting on it?!) What physics are involved in this?
Thanks :)

Related Other Physics Topics News on Phys.org
Integral
Staff Emeritus
Gold Member
There are more forces then just gravity at work. The atmosphere exerts a force as well it and this is the force which keeps the water in the bottle.

Tide
Homework Helper
Integral said:
There are more forces then just gravity at work. The atmosphere exerts a force as well it and this is the force which keeps the water in the bottle.
But not for too long - supporting a heavier fluid with a lighter one is intrinsicaly unstable (Rayleigh-Taylor!).

Integral
Staff Emeritus
Gold Member
Humm... Humming bird feeders, pet rodent water bottles all seem to hold the water for on demand release. As long as the seal is good they retain water nearly indefinitely.

Clausius2
Gold Member
Solidmozza said:
Today when I was toying with a water rocket I came upon a strange phenomena. The setup was like this:
I had a 625 mL coke bottle, filled half way with water. The cap had a nice hole in it, about 1.5 cm. I turned the bottle upside down, thinking that the water would just gush out, but low and behold it kept in there, only dripping once every 4 seconds or so. I tried this again, and the same result occurred
So heres my question, why did it do that? Why didnt the water just gush out at once (like it should with the force of gravity acting on it?!) What physics are involved in this?
Thanks :)
First of all it seems to me that the hole is too larger for experimenting what you've said. Anyway, the water does not gush out due to the surface tension. This phenomena is typical in hydrostatics and fluids at rest, and when certain ratios between dimensions, capillarity forces and gravity forces are reached. This figures are measured by the Bond Number:

$$B_o=\frac{\sigma}{R^2 \rho g}=\frac{72.8\cdot 10^-3}{(1.5\cdot 10^-2)^2 1000 \cdot 9.8}=0.03$$

It's roughly of order 1, so that I've said I think your hole is too larger. Anyway, the force of surface tension will be approximately of the order of gravity forces in the neighborhood of the hole.

The surface tension acts like a solid membrane in the water surface. You have to push down this membrane in order to get through it. This force is balanced with the gravity force. For example, when you fill an extremely narrow pipe with water, leaving is both sections opened, and put it upside down, the fluid does not flow across the pipe due to the gravity force. By contrast, the force that counteracts the gravity is the surface tension. If you see carefully the bottom opened section of the pipe, you will see a slight curvature of the surface. This curvature balances the gravity force, "stressing" the water surface itself in order to avoid the fluid flow.

Yeah It was smaller than that, I just realised (i was estimating :tongue2: ) but thanks for that Clausius, I knew something was up!
Thanks for all the quick replies!

Moonbear
Staff Emeritus