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Wieght and Gravity problem

  1. Aug 31, 2006 #1
    As part of a greater problem, i wonder if anyone could help me with this one. If i had a 100 meter length of pipe laid on the ground that wieghed say 100kg. if i stood the pipe up would the pipe wiegh less, allowing that the force of gravity decreases with the square of the distance from the earth if so how would i calculate the new wieght. I hope someone can help thx.
     
  2. jcsd
  3. Aug 31, 2006 #2

    Doc Al

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    Unless the purpose of the exercise is to test your ability to integrate, for all practical purposes the strength of gravity near the earth's surface can be considered uniform over 100 meters. The weight will be the same regardless of orientation.

    What's the exact problem you are trying to solve?
     
  4. Aug 31, 2006 #3
    the exact problem is a lot more complex and i know some one out there would maybe solve it easly, but if i can i would like to solve it myself with a little help. i used the length 100m and weight 100kg as arbitrary units easy to deal with. You say "for all practical purposes the strength of gravity near the earth's surface can be considered uniform over 100 meters." What if my pipe were 100000mtrs?
     
  5. Aug 31, 2006 #4

    Doc Al

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    There would be about a 3% drop in field strength over that distance. The strength of the field is inversely proportional to the square of the distance from the earth's center.

    If you state the exact problem, then it might become clear what's important and what's not.
     
  6. Aug 31, 2006 #5
    thankyou Doc Al the exact problem goes like this. I was reading the New Scientist this morning and found this article http://www.newscientistspace.com/article/mg19125671.900;jsessionid=GBBIBOODMNDI
    If you cant get the full article let me know and i;ll post it.
    Any way here goes, if i knew what the length of cable was, its density and the gravitational decrease i would know its "weight". Now here is the problem i have created. If i could find the "weight" of a few different materials along with their tensile strengths etc i could maybe come up with a formulae to find the exact characteristics that would meet the requirments of the cable(maybe).
     
  7. Aug 31, 2006 #6
    You have a 100m long solid (not hollow) pipe and when you put it on the Earth's surface horizontally it weighs 100kg. Technically, if you put the same pipe vertically on the Earth's surface, its weight should go down (by an extremely small amount), right?
     
  8. Aug 31, 2006 #7

    Doc Al

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    Of course.
     
  9. Aug 31, 2006 #8

    Doc Al

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    I could only see the beginning of the article, but it sounds a lot like the "skyhook" problem, which has been discussed a bit here on PF (and plenty of other places too). Do a search on "skyhook" (or "sky hook") as well as on "space elevator" and you might find some interesting stuff.

    Here's one thread I found: https://www.physicsforums.com/showthread.php?t=74592
     
  10. Aug 31, 2006 #9
    thx Al i'll be back
     
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