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PostPosted: Sun Feb 05, 2012 11:56 am 
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One of the projects I am working on now and then is a pseudo-historical / fantasy
setting where a dimensional gate enables scholars from Earth to visit another world.
This world has a slightly lower surface gravity than Earth. The visitors will probably
feel a difference, but do they have a way to measure the difference with medieval
scientific instruments ?

A scale and weights will probably not be sufficient, because the relation between
the object weighed and the weights used remains the same, independent from the
gravity. Is there anything else that could be used successfully ?

Any ideas would be most welcome. :)


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PostPosted: Sun Feb 05, 2012 12:01 pm 
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Drop a ball, see how fast it falls? That's the time-honoured way of measuring gravity in physics classrooms :)

Or see Experiments 3 and 4 here:
http://motivate.maths.org/content/gravity-experiments-0

I guess the tricky part will be accurately measuring the time it takes for the ball to fall/pendulum to swing. But that could be mitigated by doing the experiments many times and taking the average.

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PostPosted: Sun Feb 05, 2012 12:20 pm 
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EDG wrote:
Drop a ball, see how fast it falls? That's the time-honoured way of measuring gravity in physics classrooms :

Thank you very much, I really should have remembered this one
- especially because the players certainly will ... :oops:


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PostPosted: Sun Feb 05, 2012 4:05 pm 
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Time the swaying of a long pendulum. The period is independent of the mass of the bob, it depend on only on the length of the pendulum and the acceleration due to gravity. If your timepiece isn't very precise you can time a thousand swings and divide by a thousand, etc, to improve precision.

You do, however, need an accurate tape measure and an accurate clock.

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PostPosted: Sun Feb 05, 2012 4:22 pm 
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Agemegos wrote:
Time the swaying of a long pendulum. The period is independent of the mass of the bob, it depend on only on the length of the pendulum and the acceleration due to gravity.

Thank you very much. :D


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PostPosted: Mon Feb 06, 2012 7:02 am 
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There's also another way besides repeating an experiment and it somehow sounds like something they'd do in the middle ages...

... just drop the item from great height. Like cannonballs off of a cliff on this other world and on Earth. The savant, of course, would demand the finding of similar heights, which would be an adventure in itself to find a natural height of similar amount, then perhaps building a wooden platform to make up for the remaining difference.


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PostPosted: Mon Feb 06, 2012 9:36 am 
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Thank you for a good idea. :)


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PostPosted: Mon Feb 06, 2012 11:20 am 
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epicenter wrote:
There's also another way besides repeating an experiment and it somehow sounds like something they'd do in the middle ages...

... just drop the item from great height.

They didn't. Not until the Renaissance. Galileo Galilei was the first person to time objects falling through different distances and to discover that they accelerate. No-one before his time had a stopwatch, or anything serving the function of a stopwatch. Can't time things before they invent accurate timepieces, and you can't time a falling object on a watch that has only an hour hand.

Whether you're dropping balls or letting them swing on a string, the real problem is an adequate timepiece. I seem to remember reading that Galileo used a sort of simple water clock to time falling balls and so forth, and that he discovered the regularity of a pendulum by using his pulse as a standard of time, but since those things aren't calibrated you can only use them to detect a difference of gravity by transporting your clock. Have Rust's Earth scientists brought their electronic watches with them?

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PostPosted: Mon Feb 06, 2012 11:32 am 
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Agemegos wrote:
Have your Earth scientists brought their electronic watches with them?

I am afraid their most precise gravity independent devices to measure
time are things like timed candles or candle wicks. On the other hand,
since this setting is a fantasy - science fiction crossover, the charac-
ters could use or develop some kind of magic to deal with the problem.
Anyway, they will almost certainly not be able to measure the gravity,
only to demonstrate that it is different from the gravity of Earth.


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PostPosted: Tue Feb 07, 2012 12:48 am 
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Okay, that's challenging. There are two difficulties: accuracy and precision.

Burning candles only offers very limited precision. You can measure a quarter-hour or so, but the things simply burn too slowly for anything less to be measured. If I had to measure gravity with only a candle as a time-piece, I'd suspend a very long pendulum with a heavy bob from a high roof (indoors to prevent the wind from affecting the result) and count the swings over a period of at least several hours.

Burning candles also aren't very accurate. I guess that a modern machine-made paraffin-wax candle with a machine-braided wick is fairly standard, but the rate of burning is affected by temperature, presence or absence of wind or a draught, and oxygen content of the air. You'll have to regulate burning conditions for the candle, repeat the measurement at least thirty times to average out variations, etc. And even then you'll get gravity in metres per candle-centimetre per candle centimetre, which you can't convert to known units without a time-piece.

Anyway, I thought of a method that doesn't depend on having a time-piece, though of course it does depend on importing a different standard from Earth instead. If you have a way of propelling an object at a known speed and a known angle, you can estimate gravity from how far it goes before striking level ground. For this you need a tape measure, a cricket ball or baseball, and a mission staff member who has received professional coaching as a bowler or pitcher ((a) so that his throwing action is highly repeatable, and (b) so that he happens to know how fast he throws).

Have your explorers by any chance brought with them bows or crossbows that were tested on Earth?

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