basic physic question

crispygoat · #1 04-26-2011, 08:33 PM

water coming out of a tap narrows down as it falls because:

a) its speed increases as it falls
b) of surface tension
c) of air resistance
d) of atmospheric pressure
e) its speed increases and surface tension holds the steam together

**line6man** · #2 04-26-2011, 08:36 PM

Are you asking us to answer a question on your homework or something?

crispygoat · #3 04-26-2011, 08:47 PM

Quote:

Originally Posted by line6man

Are you asking us to answer a question on your homework or something?

Yes, I can't seem to find the answer in my book. I think it's A, but I'm not sure what they mean by "and surface tension holds the steam together"

Nappa · #4 04-26-2011, 08:59 PM

Quote:

Originally Posted by crispygoat

Yes, I can't seem to find the answer in my book. I think it's A, but I'm not sure what they mean by "and surface tension holds the steam together"

Is steam suppose to say stream?

fdeck · #5 04-26-2011, 10:09 PM

To think about surface tension, suppose it was a stream of sand instead of water.

Tat2dHeart · #6 04-26-2011, 10:18 PM

It's A. Go to the section on the continuity equation. The basic principle is that as the water falls, the velocity increases. When that happens, the diameter of the stream needed to carry the same volume of water decreases. Think about how rivers flow - in the wide parts, the water moves slower and as the river narrows, the speed of the water increases.

TBAR Shane · #7 04-26-2011, 10:45 PM

Google is your friend.

ehque · #8 04-26-2011, 10:48 PM

Mass flow rate (kg/s) = velocity (m/s) * linear density (kg/m)

Expanded:

Mass flow rate (kg/s) = velocity (m/s) * cross sectional area (m²) * density (kg/m³)

Mass flow rate is constant at all points along the stream, since any difference will cause mass to appear or disappear, violating conservation of mass.
Density (of a liquid) is constant, at least at constant (atmospheric) pressure.
Velocity is increasing, acceleration due to gravity.
Solve for change in cross sectional area.

I'm not sure that the mechanism holding the stream together is surface tension, although that's probably the layman's definition - i would have used "intermolecular forces" - or more specifically, hydrogen bonds - which act throughout the material rather than at the surface of it.

Jim Nazium · #9 04-27-2011, 04:59 AM

f) because it's shaped like a carrot. Duh.

i_got_a_mohawk · #10 04-27-2011, 06:29 AM

It's clearly caused by the Gaussian profile of water coming from a mains tap.

Pffft.

L-A · #11 04-27-2011, 06:41 AM

Obviously, this problem doesn't take into account that the water comes out of the tap through a mesh, which occupies roughly 40% of the exit area.

geeza · #12 04-27-2011, 12:04 PM

It's because the stream of water is approaching the speed of light, which makes it appear long and narrow to the stationary observer. You can see a similar example in the opening credits of Star Trek.

bigblondeafro87 · #13 04-27-2011, 12:23 PM

ehque has got your answer, but I gotta ask what class this is for? This is more a fluid mechanics problem than a physics problem...

i_got_a_mohawk · #14 04-27-2011, 01:25 PM

Quote:

Originally Posted by L-A

Obviously, this problem doesn't take into account that the water comes out of the tap through a mesh, which occupies roughly 40% of the exit area.

Of course, so we have to worry about the polarisation of the water in this instance.

Quote:

Originally Posted by bigblondeafro87

ehque has got your answer, but I gotta ask what class this is for? This is more a fluid mechanics problem than a physics problem...

I'd say fluid mechanics/dynamics is physics based, and not even just in the arsey way that "everything is physics based".

WyrmDL · #15 04-27-2011, 02:27 PM

It's definitely a physics subject, as it is heavily covered in Heat and Waves.

tastybasslines · #16 04-27-2011, 02:30 PM

The vertical approach of the optical illusion provides you with the innermost conception in the utmost vitality.

crispygoat · #17 04-27-2011, 03:07 PM

It's actually for a Physics for Non-Scientists course.

ehque · #18 04-27-2011, 09:26 PM

Quote:

Originally Posted by crispygoat

It's actually for a Physics for Non-Scientists course.

I'm not sure then if the mathematical answer above that I posted - which is correct, mind you - is too complicated for you?

If it is, consider the case where you squeeze the tip of a hose, reducing cross sectional area. The speed of the water increases. To solve your question just look at it the other way around, velocity has increased due to gravity, so the cross sectional area has to fall.

bassist11 · #19 04-27-2011, 09:55 PM

Funny. I'm in Fluid Dynamics class this semester. ehque's answer looks pretty sound.

Think about the Bernoulli Principle as well (which would have it's part in the continuity equation like Tat2dHeart said). The faster a liquid goes, the less pressure it has. Less pressure means a constant atmospheric pressure around the stream will push it together more.