A reader asked a columnist why water draining from a washbasin rotates
counterclockwise in the northern hemisphere and clockwise in the
southern hemisphere. The columnist answered: "That's hooey!" but
didn't explain. I'd like to find a labeled vector diagram or 3-dimensional model
for better understanding.

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Request for Question Clarification by pafalafa-ga on 29 Jul 2004 20:34 PDT

Water will drain according to the contours of the basin and perhaps
any residual momentum imparted by the initial flow of the water into
the basin.  There are no discernible effects of the so-called Coriolas
force on such a small volume of water.

Do you really want vector diagrams, etc. showing the effect of sink contours?

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Clarification of Question by yevok-ga on 02 Aug 2004 19:11 PDT

Please read the comment from yevok posted 8/02/04. Since this is my
first use of Google Answers, I didn't know there is a formal
clarification process.

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Request for Question Clarification by hedgie-ga on 02 Aug 2004 23:17 PDT

You can cancel (expire) the question if it was (as it seems it was)
answered in comments and references. 
If it is still a question, perhaps you can clarify what diagram you
are seeking, diagram of negligeable forces due to rotation of Earth
or diagram which may clarify how shape of basin etc may affect the spin?

A similar question:
http://answers.google.com/answers/threadview?id=323911

I believe that in the Northern Hemisphere winds blow anti-clockwise in
a low pressure area and clockwise around a high pressure area.
In the Southern Hemisphere the winds blow Anti-clockwise around a high
pressure area and clockwise around a high pressure area.

This may have something to do with it

Kind regards 
Ross

http://www.snopes.com/science/coriolis.htm

The Coriolis Force has to do with the idea that because the earth is
spinning, a system moving from the poles towards the equator will
experience a lateral force on it and vice versa from the

At the poles, there is no centrifugal force from the earth's spin
(radius = zero), while at the equator, there is a centrifugal force
(radius = 6970 km).

So, on a large scale system, like the ocean, the part of water closer
to the equator is expierencing a relatively larger lateral force,
while the part nearer the poles is experiencing a relatively smaller
lateral force.  Thus, you get a rotation.  In the northern hemisphere,
for instance, low pressure systems travel counter-clockwise.  You can
look at satellite images of low pressure systems, hurricanes being
extreme examples, and they all go CCW in the northern hemisphere.

However, it does not effect small scale systems like your sink,
toilet, etc.  Even most lakes are immune to the coriolis force,
because the internal friction between the water molecules (viscosity)
as well as the friction between the water and the basin is stronger
than the coriolis force.

Doing the math,  centrifugal force's equation is F = m*v^2/r.

Divide this by 1/4 the circumfrence of the earth (distance from equator to pole). 

This amounts to about 1/2 newton per kilogram over a distance of ~11000km.

 Your sink, which is probably (45 cm) 18 inches across, (assuming
across means N-S -- and that's a big sink)has a force gradient of 2 x
10 ^-8 N between the N and S side of your sink.  Hardly enough to make
a difference.

But then again -- this issue was the topic for an episode of The
Simpsons, and if it's on TV, it's gotta be true!

Let me rephrase my question. I want to know what force or (?) cause
the rotation of the water draining from my four washbasins in the
counterclockwise direction. If the answer is complex, a diagram with
labels is always helpful especially since I want to explain it to
another. One other comment. You used the word coriolis. I thought the
coriolis force might be the answer since it works in the opposite
direction in the two hemispheres. However, is seeking more information
I found that although the coriolis force on the north and south sides
of my washbasin differs, the force is so small as to be nearly
unmearsureable.

In my house, I have four washbasins. They have a circular shape
flattened on one side with the drain an inch away from the flattened
edge. I filled them all slowly and let them sit for an hour then
drained them and the water rotated counterclockwise in all four.
Obviously a force is at work here and I'd like to know what it is.

With the way water works, I know that there can be lingering effects
of how the sinnk was filled up to 12 hours after the tank was filled. 
Perhaps one hour hold is not sufficient.

One way you can test out what's going on is to fill the basins using a
bucket.  On one, fill it up so that the water swirls in a CW direction
and CCW in another.

Let it sit an hour and drain.  I suspect you will see the water drains
in the direction you filled it.

Also, try waiting 24 hours before draining and see what happens.

All this discussion is making me doubt a foundation I was previously
so sure about.  I am a HS physics teacher, who has traveled to
Australia and I have no reason to doubt the accuracy of the simpsons
and the coriolus effect in the loo.

I do belive the spinning of the water is due to the Coriolus effect. 
I have tested this hypothesis before and even if you start the water
spinning the wrong way, it can change direction and spin back in the
previous direction.  Furthermore, it spins the opposite direction in
Australia.

Some people doubt that the small force can turn water in a sink but I
would contend that the rotation of the earth can be witnessed in the
circular nature of a pendulum.  Also, balloons rise due to air
pressure differences between the top and bottom of the balloon, this
too is a small force that causes noticable effects.

The coriolus effect in storm systems is due to the fact that the side
toward the equator has a greater rotational circumference than the
side nearer the pole.  Because that side is moving further it causes
the storm system to spin.

Finally, he may have been pulling my leg but a well-traveled college
evironmental science teacher told me that when near the equator, water
just drains out.  This fits with the coriolus effect.

bariot - Your correct about the coriolis effect and the effects of
small forces or (pertubations) can have on objects. However this is a
little off subject.  Balloons rise because of buoyancy - EG: The fluid
in the balloon is less dense than the ambient fluid.  The atmospheric
boundary layer is to turbulent to be quiescent - which is a necissary
condition for your explanation.

If you had the same fluid in the balloon as in the ambient fluid, then
the balloon also would rise in your explanation due to the pressure
gradient from gravity.  When in reality it would fall.

Just a friendly comment. =)

bariot--what is it going to take to convince you that the direction of
rotation of water going down the drain has nothing to do with the
rotation of the earth?

The notion of water spinning in a sink as due only to sink conditions
is heartbreaking.  I'm thinking that this idea is typically tested
with cold water and the tap is on that side so... Okay, I bite.  But
now I feel I had better e-mail some students and apologize for
spreading this false information.

I would still like to readdress Saem about balloons.  Just as I was
sure about the draining water, I find many scientists sure about
balloons rising but for overly simplified and misunderstood reasons. 
In a faulty explanation a "buoyant" force is often referred to, and
"lighter than" is cited as important but these facts are improperly
strung together.

There is no such thing as a buoyant force as a type of force, just as
there is no such thing as a centripetal force as a type of force. 
Merely, these are the overall result/sum/direction of forces in
particular systems.

The buoyant force is a net pressure difference between one surface of
an object and the other.  This normal force is equivalent to the
weight of the fluid displaced.

A basketball experiences the same "buoyant force" as a similarly sized
balloon.  But, because the basketball is heavier than the balloon, the
weight force down is greater than the buoyant force (net pressure,
normal force) up so the basketball falls.  In the case of the balloon,
the weight force down is less than the "buoyant" force acting up so it
rises.

For similar reasons, balloons in your car move from the back seat
toward the front when you step on your breaks.  In this instance, the
air molecules rush to the back of the car and push the balloon
forward.

Using this model, can you think of what happens to a helium balloon in
a free falling elevator?  How about an elevator that is accelerated
downwards faster than gravity?  In both these cases, the balloon is
still less dense than air but but in neither does it rise.

Take care.

Hi bariot--

You mean when you step on the brakes, the balloons go to the back of
the car.  When you accelerate they go to the front.

Also, you are right that the buoyant force is due to the difference in
pressure, but I don't see anything wrong with calling it a buoyant
force.  You mentioned a 'normal force' in your comment, but if there's
no such thing as a buoyant force, there's no such thing as a normal
force.  There are just four types of forces.  When you get right down
to it, the force holding a balloon up is electromagnetic in
origin--electromagnetic forces are what keep the air molecules from
passing though the balloon.  But buoyant forces and normal forces are
higher level concepts, and are useful even if they are not
fundamental.

Third, I think when you say there's no such thing as a centripetal
force, you mean centrifugal.  There are centripetal forces--they are
the inward forces that cause objects to follow circular paths.  The
gravitational force the sun exerts on the earth is a centipetal force.
 The centrifugal force is a pseudo-force felt by an observer
undergoing circular motion.

"You mean when you step on the brakes, the balloons go to the back of
the car.  When you accelerate they go to the front." -racecar.  

Yes that is correct because a pressure gradient forms due to the cars
acceleration! =)

Oddly the same question with regard to the balloon is often on PhD
exams in engineering!

Racecar has it on the money.

Go racecar with your 4 forces comment!

It's true.  If you want to get in the nitpicky bogged down details,
there are only four forces in this universe:  Gravity,
Electromagnetic, Strong Nuclear, and Weak Nuclear.  Every force on
this planet you can think of is a result of one or a combination of
these forces.

So let's please not start with all this talk about "there's no
centripetal force, there's no buoyoancy force, blah blah blah"  These
forces may not "exist", bit it's sure easier to say "a buoyancy force
of 1 N" instead of "a pressure differential of 1 N/m^2 over an area of
1 N/m^2"  But wait, you can't have pressure, because that's
force/area, right?  What I am getting at is to forget the naming
conventions for a second and look at the actual physics involved.

Balloons rise because their mass is less than the mass of the volume
they displace of fluid in which they are immersed.  This is the same
reason why boats float, by the way.  This is also the reason why oil
"floats" on top of water.  In other words, a balloon floats, because
it is less DENSE than its surroundings.  Since its surroundings are a
fluid (gases are fluids), then the balloon rises.

Next, let's discuss relativity and inertia.  Let's take this from the
perspective of somebody OUTSIDE the car.

When you accelerate in a car, things don't move backwards.  Things
stay in place, due to Newton's first law, stating an object in motion
(at rest) stays in motion (at rest) until an external force acts on
it.  So, when you gun it, all the loose crap in your car stays in
place.  Rather, your CAR moves forward.  To somebody inside the car,
it would seem that stuff is moving backwards.

When you slam the brakes, the opposite happens.  Everything else not
bolted down in your car continues to move at your previous speed. 
Your car, however, slows down.  To somebody inside the car, it would
appear that things are moving forward.

Everything not bolted down includes air molecules.  So, the air
molecules also move towards the front of your car.  This creates a
partial vacuum in the rear of your car (aka a positive pressure
gradient in the front of your car).  This exerts a force on everything
in your car to move it to the back.  This force is tiny, however, so
for most things, friction takes over and holds boxes of kleenex, soda
cans, etc in place.  Something like a balloon, however, does not
require much force to move large distances (recall force = ma;
distance travelled if intial relative velocity is zero = 1/2at^2). 
Since the balloon has such small mass (less than that of air of the
same volume), and our force is constant, then we get a relatively
large acceleration.  Therefore, we get a relatively large (visible to
the eye)distance travelled.

It's really that simple.

Regards,

-touf