MadSci Network: Physics

Re: Why does water circle around a drain instead of running in a straight line

Date: Sun Dec 10 15:11:04 2000
Posted By: Vernon Nemitz, , NONE, NONE
Area of science: Physics
ID: 975598439.Ph

Greetings, Sarah:

Water swirls down a drain because Planet Earth is rotating.  Possibly,
one could go so far as to say it is proof that the world turns.  But
to derive such an uncommon statement from such a common event is going
to take some explanation....

Get yourself an ordinary ball and set it on a table.  Place one finger
on top of the ball to hold it in place.  Now take a marker and put some
dots on the ball, in a nice vertical straight row, from near your
finger, down toward the table.  Finally, keep your finger in place
while you turn the ball slowly, so that the vertical row of dots stays

After one complete turn of the ball, take a moment to think about how
far each dot moved.  Each moved in a circle, but the ones near your
finger, or near the tabletop, moved in small circles, while the ones
near the middle of the ball moved in larger circles.  Now, since each
dot took the same amount of time to move in its circle, how fast did
each dot move?

It should be obvious that the dots that moved in the larger circles had
to move faster.

Now think about the Planet Earth:  It is like a ball, only bigger.
The "middle" of this ball is usually called the Equator; it is a circle
more than 24000 miles in circumfrence.  Next, since there are 24 hours
in a day, it is plain that for a "dot" on the Equator to move more than
24000 miles in 24 hours, it must move at over 1000 miles per hour.
Meanwhile, a "dot" located near either the North Pole, or the South
Pole, will move very little in 24 hours.


Time to switch subjects for a bit.  Take a look at a music disc, and
consider the small circles near the hole, and the larger circles near
the rim.  You know that the whole disc turns together, so just like the
ball, a dot on the rim has to move faster than a dot near the middle.

Next, think about some astronauts orbiting the Earth.  It is easy to
imagine them going around the world like a dot on the rim of a disk.
BUT:  suppose we look very closely inside the astronauts' spaceship;
some parts of the spaceship are closer to the Earth than others, right?
If you were to place two dots inside that spaceship, the one closer to
the Earth would go around the Earth just a little bit slower than the
dot located farther from the Earth, wouldn't it?

Whenever any object, such as the spaceship, contains spots that have to
try to move at two different speeds at the same time, there will be
some stress in that object.  If you try to spin that music disc too
fast, it will fly to pieces.  Fortunately for the spaceship,  it only
experiences some very small stress.  We call it "tidal stress".  It is 
sufficient to cause loose objects in a spaceship to gradually move.
Some people may erroneously refer to it as "microgravity", because of
that motion.  On the other hand, there is ALSO some miniscule amount of
real gravitation present, between those loose objects.  Anyway, it is
important to know that tidal stress is a FORCE -- if something can
cause an object to begin to move, it is always because that something
is a force.


Meanwhile, back on Earth, we have all this water, which most certainly
is a very loose collection of molecule-sized objects.  In whatever
manner you wish to describe water moving, it does so because some kind
of force acts upon it.

Note a drain, for example.  At most places on the surface of the Earth,
you can place two dots near the opening, such that one dot is closer to
the Equator, and the other dot is closer to a Pole.  As the world
rotates, one dot will be moving very slightly faster than the other.
Therefore, we have a condition similar to that inside the spaceship,
where a tiny force is present.  (In this particular case, the correct
name is "Coriolis Force".)

One of the basic facts about forces is that not only do they cause
objects to move, the longer a force is applied, the faster an object
will move.

So...when we have a tub full of water, and pull the plug to let it
drain out, there will be a tiny force causing some of the water
molecules to begin to move in a swirl.  This initial motion is far to
small for you to see, however.  BUT:  The force exists steadily for as
long as there is water in the tub to drain; gradually the swirl goes
faster and faster.  After a few seconds, you will become able to see
it; after a few more seconds, the swirl is unmistakable.  The more
water available, the longer it will take to drain, and the longer that
tiny force will have, to build up a larger and larger swirl.

One final point is relevant:  The location of the drain, on the surface
of the Earth, affects the swirl significantly.  In the Northern
Hemisphere, draining water naturally swirls counterclockwise; in the
Southern Hemisphere, it naturally swirls clockwise.  Also near either
Pole, the Coriolis Force has maximum effect upon water going down a
drain.  At the Equator -- EXACTLY at the Equator; imagine it as a
mathematical line that perfectly bisects the drain -- the Coriolis
Force is zero.  (More precisely; two Coriolis Forces are present, one
for each half of the bisected drain, but they cancel each other out.)
In this special case water SHOULD be able to go down a drain simply
straight, just as you wondered about in your Question.

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