You probably had to start the water flowing in the tube by either sucking at the end with your mouth or submerging the whole rig. This fills the hose to equal pressure. When you turn it over, gravity helps lower the pressure in the one side, so the water from the other end flows in to equalize pressure. Hope that makes sense. It's called a siphon.
Yeah, you're describing a vacuum or empty space. The pressure from the atmosphere pushes against the water to fill the vacuum and wala, it goes against gravity!
AHEM... NOT A VACUUM
but if you want to learn about vacuums, here's an interesting fellow Torricelli
He invented the barometer
Firstly... there is not initial gravitational force. Its just Gravitational force.
Also... its important to note... gravity is considered to be very weak. Electromagnetic force is 1.0e36 (1000000000000000000000000000000000000) times stronger. Its also important to note that force is a vector... not only does it have a magnitude but a direction. Earth gravity direction is toward earth.
Now Force is defined as the change in momentum over change in time
Momentum is Mass*Velocity
that being said let us begin... water is polar... it likes to stick together...
to illustrate take a water gun and shoot some at the celling.... good. see how it sticks? cool.
Now consider this. Water is a fluid. So is Air.
Bernoulli's principle states: as the velocity of a fluid increases, the pressure exerted by that fluid decreases. Now this tube of yours exerts normal force(perpendicular to surfaces) . Pressure is a force per unit area. the thinner your tube, the higher pressure the higher flow velocity.
From that we also get Torricelli's Law, which relates initial velocity, gravity, and height to the flow of a liquid out of an opening
Now think of your tank as a bunch of sheets mass stacked up on each other.
Now gravitational force points towards the "bottom" or your container. The sheets of water are being "pushed down" by the ones at the top. Thus there's more pressure. This is why your tube has to have its opening at the bottom of the container. basically there's a pressure differential occuring at the opening of the tube in the water.... which relates to velocity. and since the pressure of the sidewalls of the tube differs from that of the sidewalls of the container... there's going to be a velocity flow in that tube.
water flows out of the tube.
