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Q: Earth Rotation ( Answered,   5 Comments )
Subject: Earth Rotation
Category: Science > Astronomy
Asked by: docbob-ga
List Price: $10.00
Posted: 21 Dec 2003 08:14 PST
Expires: 20 Jan 2004 08:14 PST
Question ID: 289253
I understand that the earth rotates counter-clockwise in the Northern Hemisphere

When I go to a big science museum - they will have a pendulum that
will seemly rotate 360 degrees in 24 hours - knocking little blocks of
wood down in a circle. In reality the pendulum is staying the same,
but the earth is rotating underneath it.

I understand the above and I have two rugs in my house that constantly
seem to move to the west, actually they are standing still like the
pendulum and the house is moving underneath them:

My question is >>>>> Why dont more things in the house do the same
thing? (Magic rug is not a satisfactory answer)
Subject: Re: Earth Rotation
Answered By: andrewxmp-ga on 21 Dec 2003 12:50 PST
Hi Docbob,

It seems as though you've actually made an interesting scientific
discovery! You have an appropriate conclusion based on what you saw
the pendulum doing, and you have appropriate and seemingly supportive
evidence based on what your rugs are doing!  However, I can assure you
that your rugs are not 'drifting westward' due to the earth's

The reason that random household objects do not move as the pendulum
does in a westward fashion is that they are bound by the force of
friction to whatever they are resting on.  This force, although easily
overcome by a person moving a chair or lifting a glass for example, is
far stronger than the lateral force produced as the earth turns.  The
same question is often asked of the force of the earth's spinning on
the flushing of a toilet; is the direction influenced?  The short
answer is "no", the friction and initial direction of the water is far
more influential.  The long answer is:

" A Peculiar Domestic Question 

Does the rotation of the earth affect the way in which water runs down
the plug hole when you empty the bath? Some people say that the water
goes down clockwise in the Southern hemisphere and anticlockwise in
the Northern hemisphere. Such people have probably never, or very
rarely, looked. In some bathtubs (basins, toilets etc) and under some
conditions, the water runs out clockwise, in others it runs out
anticlockwise. There is no correlation with the hemisphere. Other
effects may lead to the direction of draining. For instance, some
basins have separate cold and hot water taps that are positioned
symmetrically left and right. If you fill the basin using the left
hand tap, you set up a rotation in one direction, and this will
determine the direction in which it drains. Using the other tap
reverses the direction. Many basins and baths are sufficiently
symmetrical that it is possible, with some care, to have the water
drain with no observable rotation. "
[!/FOUCAULT_PENDULUM/foucault_pendulum.html ]

So then, why does the pendulum in the museum remain motionless in
relation to the earth? First, this type of pendulum is called a
Foucault Pendulum, after it's scientist inventor.  Each can vary
considerably, but has a few critical features: a very heavy pendulum
weight (called a "bob"), a very long cable, and a mechanism for
minimizing friction between the cable and the ceiling it is attached
to.  The important thing about friction here is that no, a Foucault
pendulum is not perfect because it is influenced by friction with the
air it swings through and with the ceiling it is attached to, however
great care has been taken to minimize this friction:

"Instrument makers attempt to create their parts for such experiments
in a manner that will eliminate friction. Unfortunately, in the real
world that is impossible. Foucault's pendulum is attached to a ball
that rotates in a socket. It is constructed in such a way as to permit
the pendulum to freely move as the socket rotates with the building
throughout the day. However, despite all of the grease and any other
mechanical tricks, that ball and socket creates friction. That
friction will cause the pendulum to slow. It will also cause the
pendulum tension will affect the path it takes. Remember, the path
needs to be perfect in order to attain the exact starting path after
twenty-four hours.
Additionally, the pendulum swings through the air. As it does so, the
air emits a force upon the bob and wire. This constant friction also
causes the pendulum to sway from its perfect path. After twenty-four
hours, the pendulum does not appear to be traveling in the same path
it once was. "
[ ]

So, the pendulum has had special engineering steps taken to reduce
friction as much as possible, and it is still noticeably inhibited
from moving perfectly free from the earth.  The objects sitting around
your house, especially your rugs with their large surfaces areas, have
extremely high frictional contact with your house and thus the
spinning earth.  They would certainly not be expected to move. 
Imagine how crazy our world would be if things started sliding around
at random!

Final question: why IS your rug moving westward?  Is it migrating? 
Does it like the west side better?  Are there rug ghosts in your home?
 I'm not really sure.  It is most likely due to some other physical
influence, such as drafts of air conditioning air getting underneath
them.  Maybe the floor underneath is highly waxed?  Beyond that I
cannot guess, but I do know that the earth's rotation is not the

I hope this information has sufficiently answered your
question.  If you have any further questions about this, please
request an answer clarification, especially before rating this answer.
 Thank you for bringing your question to Google Answers!


Search terms used: 

24 hour pendulum
Foucault Pendulum
"Foucault Pendulum" friction
Subject: Re: Earth Rotation
From: xarqi-ga on 21 Dec 2003 15:09 PST
Is your rug on a carpet?

If it is, it will "migrate" in the direction of the carpet pile.  The
principle is the same as those clothes brushes with the one-way
Subject: Re: Earth Rotation
From: racecar-ga on 22 Dec 2003 10:14 PST
A Foucault pendulum will only make a full rotation in 24 hours if it
is at the north pole (or south pole).  Everywhere else the rotation is
less, by a factor of the sine of the latitude.  So, if you live at
latitude 30N, a Foucault pendulum would take 48 hours to make it all
the way around (though it would knock all the pegs down in 24, since
that only takes 180 degrees of rotation.
Subject: Re: Earth Rotation
From: xarqi-ga on 25 Dec 2003 22:27 PST
Hi racecar-ga:
Got a reference for that?  Frankly, I find it impossible to reconcile
with my notions of mechanics.

There are some curious, infrequently described, facets of the Foucault
pendulum, such as the total LACK of rotation if swung in an E-W
direction precisely on the equator, and the variation of amplitude
with time of day, and hence degree of rotation, and the depatuure of
the swing plane from the vertical (which ARE dependent on latitude),
but I can't swallow the pendulum taking any time other than 1 day to
return to the same relative orientation to the local frame of
reference (the equatorial situation, excepted).

The essence of the Foucault pendulum is that the plane in which it
swings is independent of the rotation of the Earth.  After 1 day, the
Earth will have returned to the same orientation relative to this
plane, no matter what the latitude of the experiment.

I'd be obliged if you could point out the flaws in my understanding.

Subject: Re: Earth Rotation
From: racecar-ga on 05 Jan 2004 15:03 PST
Hi xarqi,

Sorry for the delay.  I only just saw your comment.  Here is a reference:!/FOUCAULT_PENDULUM/foucault_pendulum.html

In the earth reference frame, motions are influenced by the Coriolis
force, a pseudo-force that arises due to the rotation of the earth. 
The Coriolis force is proportional to the sine of the latitude, and it
is this force which causes the precession of a Foucault pendulum.

Even though the earth rotates once a day, the rotation rate of the
local reference frame is not once per day everywhere on earth.  In
fact, this rate of rotation only occurs at the poles.  If you
represent the rotation of the earth as a vector, then this vector is
everywhere the same: it is oriented parallel to the axis of the earth,
and its magnitude is the rate of rotation of the earth.  The rate of
rotation of the local reference frame is the component of this vector
which is in the local vertical direction.  In other words, it is the
dot product of the earth rotation vector with the local unit normal
Subject: Re: Earth Rotation
From: xarqi-ga on 05 Jan 2004 17:14 PST
Hi racecar-ga!

Wow!  Thanks.  The more I learn, the more I realise how ignorant I am!
This new (to me, that is) information is right up there with the
direction of bath water rotation NOT being tied to hemisphere!

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