Why do bottles in a sink persistently float towards the stream of water rather than drift away?

This message has been edited. Last edited by: DorianGreyed,

Your intuition may be that faster moving water exerts greater pressure. It does, but only in the direction of the flow. To the side of the flow, faster moving water results in reduced pressure; thus, your objects are drawn to the water stream.

The same is true of air. Airplane wings are designed so that air moves faster above them (creating a lower pressure) than below them, providing lift.

[This message was edited by coldfuse on 04-30-03 at 04:25 PM.]

A few reasons.

1) The fast-flowing water drags the air around it down with it. This means that the air around the water will have a lower pressure.

We actually use this in chemistry labs. We use a tube with one end at next to the fast flowing water, and the other end inside a flask. On top of the only other opening to the flask, we place a filter with incredibly small holes. Normally, it would take a really long time to get anythign through the filter because the holes are so small. But when we turn on the water, the lower pressure around the water sucks the air from the flask. The lower pressure in the flask then sucks liquids or air through the filter.

2) When the falling water hits the surface and then falls below the surface, it drags the water around it with it. This water drags the water around it, which drags the water around it, etc. This results in the water on the surface flowing towards the center. This matters most for deep sinks.

3) If the bottle is just the right shape and is floating low enough that it won't can't be rocked easily, water hitting the side closest to the falling water will curve around it and flow off on the other side. This propels the bottle further into the fallign water (like when you sit on a chair with wheels and throw a ball and are propelled in the opposite direction). If the bottle rocks, the force of the water hitting it will knock it around and it may be propelled out of the stream instead.

Jeeze you smarties and i thought it would have a simple answer! thanks you two!

"Airplane wings are designed so that air moves faster above them (creating a lower pressure) than below them, providing lift."


I'm afraid this is a myth. It is similar to the myth of water spinning in a certain direction in a sink in both its popularity and in the fact that it takes a real effect and uses it to try to explain something that it is not powerful enough to do. Just like the swirling myth, this one is convincing enough that it has ended up in some high school physics texts books. It has even made it into some pilots manuals. The myth is based on the assumption that the air above and below the wing has to move at the same speed. Due to the shape of the wing, the air above the wing has to travel faster to do this. The faster moving air has less pressure, so the plane goes up.

Major problem, the air above the wing does not have to reach the back of the wing at the same speed as the air below the wing (in reality, the air above the wing actually reaches the back of the wing faster).

To see that the wing shape can't be primarily responsible for lift, think about the fact that many planes can fly upside-down as well as they can fly right-side up. Also realize that paper airplanes fly because of lift, but don’t have specially shaped wings. Finally realize that some planes, especially those designed to fly upside down best, have wings that are shaped the same on the top as the bottom.

So what is the real cause?

When you see most planes flying, notice that their wings aren’t parallel to the ground. They are angled slightly upwards. This is known as “angle of attack.” To get more lift (the force holding the plane in the air), you increase the angle of attack. Angle of attack is the most important variable in how much lift is created. If it were just about the speed the air is moving at, this wouldn’t increase lift, but it does.

The reason behind this is that the air hitting the underside of the wing will bounce off at an angle that is partially downward and partially backward. This will propel the plane upward and forward (again, the example of throwing a ball while sitting in a chair on wheels).

The shape of the wing is not to make the speed of the air different above and below the wing, it is to direct the air downward. This shape means that the angle of attack does not have to be as steep as it would otherwise have to be to get the same lift. Even without specially shaped wings, planes can fly at sufficiently steep angles of attack.

[This message was edited by methos5000 on 04-30-03 at 05:47 PM.]