# A Question of Physics 101

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• posted on April 20, 2007, 5:46 pm
Since I've been enjoying the topics and discussions of this news group, and I see that there are many posters who are quite bright, I decided to pose a question concerning the laws of physics to stimulate the group's curiosity.
The question is: If you hang two plumb bobs, say 50 feet apart are the strings parallel?
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• posted on April 20, 2007, 5:54 pm

I would say they are as close to parallel as one would care for them to be. If you want to get technical, they are not exactly parallel due to the curvature of the earth. One would assume that the bottom of the plumb bob is pointing at the exact centre of the earth due to gravity and since we're on a ball, they wouldn't be exactly parallel. At 50 feet apart, the difference wouldn't be enough to care about.
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• posted on April 21, 2007, 2:28 pm
efgh wrote:

of my teachers explaining that the uprights had to be 6" out of parallel due to the curvature of the earth. I don't know if the number is accurate, but that is what I recall (after forty years or so).
Glen
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• posted on April 21, 2007, 2:51 pm

Let's do some quick math.
The towers are about 700 feet tall, and the Earth's radius is about 4000 miles. 700 feet / 4000 miles is about 1 part in 30,000.
Since arc length is proportional to radius, the tops of the towers are 1 part in 30,000 further apart from each other than their bases. The towers are about a mile (call it 5000 feet) apart, so the tops are about 5000 / 30,000, or 1/6 of a foot further apart than the tops. Call it two inches.
Now, let's got to the video tape. Wikipedia says:

Not bad for back of the envelope, huh?
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<%-name%>
• posted on April 21, 2007, 3:06 pm

You're a star. :)
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<%-name%>
• posted on April 21, 2007, 5:35 pm

Ding Ding Ding Ding. We have a winnah!
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• posted on April 21, 2007, 6:08 pm

Also, the plumb bobs contain mass, and exert gravitational attraction upon each other. That would further take the strings out of parallel.
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• posted on April 20, 2007, 5:54 pm
For woodworking - parallel enough. For scientific purposes - not quite. This is assuming, of course, that you're doing this on the earth's surface.
GROVER wrote:

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<%-name%>
• posted on April 20, 2007, 5:58 pm

Theoretically, no, but I doubt if you could measure the difference if they were just 50 feet apart. If the 2 bobs were, say, a couple thousand miles apart, they'd both be pointing towards the center of the earth, so they definitely wouldn't be parallel. Isn't that the principle that led to the first calculations of the earth's diameter? Some guy looking down wells... OK, google tells me it was Eratosthenes, in Alexandria. Andy
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• posted on April 20, 2007, 5:59 pm

You guys are right on the money. I'll have to thinks up a harder question if I hope to stump the group.Joe G
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• posted on April 20, 2007, 6:35 pm
GROVER wrote:

table saws were adjusted to tighter tolerances than the differences between those two plumb bobs. :-)
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• posted on April 20, 2007, 8:16 pm
wrote:

So *that's* what's causing my kickbacks??? The earth's curvature? Knew it had to be something.
jc
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• posted on April 20, 2007, 6:09 pm
Ok then... In recent news; *** In a recent study, mathematician George Sparling of the University of Pittsburgh examines a fundamental question pondered since the time of Pythagoras, and still vexing scientists today: what is the nature of space and time? After analyzing different perspectives, Sparling offers an alternative idea: space-time may have six dimensions, with the extra two being time-like. ***
So if I turn a six dimensional bowl on my quantum lathe, primarily taking advantage of the additional time-like dimensions, how long do I have to work the finish in before it dries?
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• posted on April 20, 2007, 6:38 pm
Russ wrote:

That one is ridiculously easy..... It depends on what rpm you're running the lathe.
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<%-name%>
• posted on April 20, 2007, 7:11 pm

Did you soak it in LDD?
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• posted on April 20, 2007, 9:03 pm

Since you are probably working in imaginary time, I'd say the finish was on the piece even before your first cut. So don't worry about it.
dwhite
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• posted on April 20, 2007, 9:42 pm

The finish goes on FIRST, fool.
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• posted on April 20, 2007, 11:01 pm
Russ wrote:

The answer will differ in each parallel universe.
--
It's turtles, all the way down

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• posted on April 21, 2007, 2:11 am
42
--
Lloyd Baker
"Russ" < snipped-for-privacy@gmail.com> wrote in message
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• posted on April 20, 2007, 6:10 pm

Two answers: A1) No, since each will hang toward the center of the earth (ignoring gravitational pulls of other less massive or more distant bodies).
A2) Yes, if the correctness of the answer is going to be determined by empirical methods using measurement tools available to the average homeowner.
--
Alex -- Replace "nospam" with "mail" to reply by email. Checked infrequently.

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