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 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 rotation. 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. " [ http://www.phys.unsw.edu.au/PHYSICS_!/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. " [ http://www.fi.edu/time/Journey/Pendulum/foucault_pendulum_why.html ] 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 cause! 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! Regards, Andrewxmp 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 bristles.```
 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. xarqi```
 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: http://www.phys.unsw.edu.au/PHYSICS_!/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 vector.```
 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!```