Why is a falling column of water thinner at the bottom?

[tempo]

Why is a falling column of water thinner at the bottom?

 

I don't understand why this is...

[Kedas]

You could prove it with some math but basically it's just because the speed of the water is increasing while it's falling and since the flowrate is the same for every hight the volume at that hight must decrease.

The cohesion force makes sure the water stay's close together in the cases that it doesn't then there will just be (more) air between the water drops.

 

One more thing if it really would be ONE column of water released at one time and falling (not streaming water)

then all molecuse would fall at the same speed and you wouldn't have this effect.

[5614]

What I once observed when dropping water off a ledge which dropped for several hundred meters was that initially the water was just a big drop (or two) and then as it gained speed it would explode into many many droplets of water.

 

I assume this has something to do with the water gaining energy (due to acceleration due to gravity) and at the critical point the energy was sufficient to overcome much of the surface tension and attraction between H2O molecules at which point it 'exploded' into many smaller drops.

 

Obviously this wouldn't occur in the small distance between the tap and the bottom of the sink, but this thread just reminded me.

[Kedas]

I assume this has something to do with the water gaining energy (due to acceleration due to gravity) and at the critical point the energy was sufficient to overcome much of the surface tension and attraction between H2O molecules at which point it 'exploded' into many smaller drops.

The constant collision with air made the water break up.

[5614]

So what was it that made the critical moment critical?

 

Did it just happen to hit a big air molecule?

 

Or was it that after enough collisions with air molecules the water had gained enough energy to break it's own surface tension?

[tempo]

You could prove it with some math but basically it's just because the speed of the water is increasing while it's falling and since the flowrate is the same for every hight the volume at that hight must decrease.

 

Ah I understand, thank you.

 

But why does the flow rate have to be the same at every height?

 

One more thing if it really would be ONE column of water released at one time and falling (not streaming water)

then all molecuse would fall at the same speed and you wouldn't have this effect.

 

I see what you mean. That if the water molecules were connected in a column, then an increase in speed would affect the whole column of the water. But wouldn't an increase in speed increase the flow rate and the so the width of the column would have to decrease again like before???

[YT2095]

the flow rate is the same, when it 1`st leaves the pipe or platform it has a fixed flow rate in litres or mls per second, as it falls the gravity pulls it to a terminal velocity (it gets faster) BUT, the flow rate is the same as when it left.

if it starts at 500ml a second but falls at rate 1 with a circumferance of 20cms for arguments sake, and then after a while you measure the circ as being 10cms, then the rate of it falling will be Doubled Rate 2 so it`ll be a smaller circ, but traveling at twice the speed

 

of course what happens when terminal velocity (for water) is reached it that it`ll fragment into droplets as the friction of air molecules against the very motile water will create oscillations (turbulance) that will "Shatter" the structure until an equlibrium is reached between speed and droplet size and the turbulance is at a level the surface tension can contain.

 

hope that helps too

[5614]

hope that helps too

Yes it did, cheers

[swansont]

Did it just happen to hit a big air molecule?

 

 

Yeah, a biiiig air molecule. You wouldn't want to breathe that sucker in!

 

I imagine there's a point where the column can't get any thinner - it's at an energy minimum - and going faster means you can't have a continuous stream anymore.

[Kedas]

But why does the flow rate have to be the same at every height?

You can put a cylinder around the stream' date=' all the water that goes in the cylinder is also coming out of the cylinder and since the cylinder isn't collecting or creating water in any way the flow rate must be the same at the beginning and the end of the cylinder.

 

I see what you mean. That if the water molecules were connected in a column, then an increase in speed would affect the whole column of the water. But wouldn't an increase in speed increase the flow rate and the so the width of the column would have to decrease again like before???

Assuming that it would remain its column shape despite the collision with air molecules:

The speed will increase and so also the 'flowrate', although we can't talk about a flowrate since we only have one object with certain dimensions.

If you would have a lot of those 'columns' fallen after each other then the distance between them would increase together with an increasing speed making the amount of 'columns' passing per min. (flowrate) at any height constant.