PDA

View Full Version : tides, centrifugal force, equilibrium theory



Al
2003-Jun-01, 08:23 PM
I have long understood that the high tide on the far side of the earth from the moon (new moon tide) is caused by the earth in a sense being "pulled away" from the ocean waters--the gravitational pull of the moon on the earth (solid body) is greater than the gravitational pull of the moon on the far waters. Now I have come across another explanation--that the new moon tide is caused in part (or even totally, according to some sources), by the centrifugal effect of rotation around the common gravitational center of the earth-moon system.
One site is a slide show with explanations that didn't make sense, at least in my quick read-through. Go to http://soconnell.web.wesleyan.edu/ees106/lecture_notes/lecture-tides/HTML%20Presentation%20folder/index.htm , scroll about 1/4 down, click on "Tides are Tricky--Equilibrium theory". This site claims that new moon tides are totally caused by the centrifugal effect. That I highly doubt. Another site at http://www.co-ops.nos.noaa.gov/restles3.html goes into mathematical detail of the issue. See also http://ourworld.compuserve.com/homepages/BMLSS/Tides.htm or
http://www.solent-fishing-guide.co.uk/Information/TidalVariations.htm , which are the same, and http://schulen.nwn.de/watt/englisch/tides.html .

I had never heard of this explanation for new moon tides. When I first came across it last week, I figured I'd found an example of bad astronomy. Was I right in that assumption? I'd appreciate some other views. I just now registered on this bulletin board just to get some clarification on this topic. Thank you!
-Al

wedgebert
2003-Jun-01, 08:26 PM
Just a nipick, centrifugal force doesn't really exist. It's just a fancy name for momentium

Al
2003-Jun-01, 08:41 PM
That's what I had learned in my public school days, which is why I wrote "effect" instead of "force". However, another point of view (see http://www-istp.gsfc.nasa.gov/stargaze/Sframes3.htm, for example) is that it is a force---just a matter of your frame of reference. Certainly it is ot a "fundamental force"--it is a result of inertia, or momentum.
-al

Grey
2003-Jun-01, 08:49 PM
I had never heard of this explanation for new moon tides. When I first came across it last week, I figured I'd found an example of bad astronomy. Was I right in that assumption?
I'd say yes, you are correct both in in your original understanding of tides, and the fact that an explanation for tides which relies on centrifugal force is Bad Astronomy. You can see some earlier discussion of this idea here (http://www.badastronomy.com/phpBB/viewtopic.php?t=5129) and here (http://www.badastronomy.com/phpBB/viewtopic.php?t=436).

Al
2003-Jun-23, 11:58 PM
In response to wedgebert--This site (Badscience, actually Badmeteorology) also addresses the issue of "Coriolis effect" vs "Coriolis force". Check out http://www.ems.psu.edu/~fraser/Bad/BadFAQ/BadCoriolisFAQ.html . Scroll down to the third pointing hand ( "It's not the Coriolis force, but the Coriolis effect." ) and read the argument.
-al

The Bad Astronomer
2003-Jun-24, 12:29 AM
Centrifugal and Coriolis forces are real forces; just in a non-inertial rest frame. I have no idea why people insist they are not real. A good discussion of this can be found on the Straight Dope message board (http://boards.straightdope.com/sdmb/showthread.php?s=83b27a2c9ecb71c91d3b9ddacccb7171&threadid=191792).

kilopi
2003-Jun-24, 01:00 AM
I agree with the BA, centrifictional force is real. :)

I too was looking for a Straight Dope thread, the one in Comments on Staff Reports--O wait, maybe that's my problem, it might have been in Comments on Cecil's Columns, but Chronos weighed in. I'll be back.

PS: OK, I found it, it was buried in a thread about lunar rotation (http://boards.straightdope.com/sdmb/showthread.php?s=&threadid=159592). One of the problems with using centrifictional force is, it is easy to get confused. But the same thing can be said about physics, right?