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Name: Jim
Status: other
Grade: other
Location: MN
Country: N/A
Date: 8/24/2005


Question:
I just read in "A Short History of Nearly Everthing", by Bill Bryson, about the historic measurement, in eighteenth-century Scotland, of the gravitational attraction of a mountain. Suppose there is a large mountain next to a broad plain. I wonder how difficult it would be to measure the deflection of a plumb bob by the mountain.

I am unable to find any information about how to perform such an experiment, or whether it is beyond the abilities of a high school student who wished to do it as a science project.


Replies:
Sensitive instruments exist for measuring the acceleration of gravity at a given location. However, these instruments would be too costly and the analysis too complex for a high school science project. It is possible to make "gravity maps" of the earth -- that is acceleration of gravity vs. longitude / latitude / altitude. These measurements are sensitive to thousandth's of the nominal acceleration of gravity. The measurements are called "gravity anomalies". The most sensitive of these devices are found in satellites in a NOAA/NASA project called GRACE (Gravity Recovery And Climate Experiment). This is a powerful experimental setup because it provides data not only on position but is able to survey the entire Earth on a relatively short time scale -- something that has not been available previously. You can find gravity anomaly maps on the web site http://www.ngdc.noaa.gov/mgg/announcements/announce_predict.html It is easy to pick out "high gravity" and "low gravity" regions on the earth. The image can be enlarged with a 'click'. There are other links on that site. In addition, a "Google" search of "gravity maps" will bring up numerous web sites with maps and other information about GRACE etc. Perhaps an interesting project would be to use some more detailed data from GRACE (I am pretty sure you can find that.) and research WHY a certain location has "abnormally" high or low gravity. The gravity distribution on the site above is not at all "obvious" and a proposed explanation could be challenging.

Vince Calder



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