Speed and Distance...

geni said:
A second is defined as "the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom at zero kelvin."
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I think that may be what I was looking for, I don't understand it much, but that might be the peice I was missing. You're saying that a second can be determined by "doing" what you describe above, which is not dependent on any of the measurements that are dependent on the second to measure. Or, something like that?
 
Brian said:
I think that may be what I was looking for, I don't understand it much, but that might be the peice I was missing. You're saying that a second can be determined by "doing" what you describe above, which is not dependent on any of the measurements that are dependent on the second to measure. Or, something like that?
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Yes.
 
So how many meters in a light second?

Actually I want to know How many meters in a light nanosecond... Sorry.

Skip it... I see the answer above 300,000 m/s
 
Atlas said:
So how many meters in a light second?

Actually I want to know How many meters in a light nanosecond... Sorry.

Skip it... I see the answer above 300,000 m/s
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299 792 458 m/s in fact
 
a_unique_person said:
Velocity is a vector, that is, it has a direction and a speed.
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Don't you mean direction and magnitude (like one of them vector in physics)?
 
geni said:
A second is defined as "the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom at zero kelvin."

A meter is defined as the length of the path travelled by light in absolute vacuum during a time interval of 1/299,792,458 of a second.

Speed is simply distance over time. so if we have something moving at a fixed speed (in this case light) we can create a defintion of lenght by measuring how far that thing travels in a certian length of time.
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"zero kelvin"? Important if true. Please elaborate, as this appears to be a lacuna in my education. Looking lovely as always!

"A meter is defined as the length of the path travelled by light in absolute vacuum during a time interval of 1/299,792,458 of a second."

This disagrees with a previous poster's definition. I hope he won't use the "BS" word with you as he did with me (what a horrid man!) but be prepared.
 
TeaBag420 said:
"zero kelvin"? Important if true. Please elaborate, as this appears to be a lacuna in my education. Looking lovely as always!

"A meter is defined as the length of the path travelled by light in absolute vacuum during a time interval of 1/299,792,458 of a second."

This disagrees with a previous poster's definition. I hope he won't use the "BS" word with you as he did with me (what a horrid man!) but be prepared.
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The phrase "at zero kelvin" does not appear in NIST's definition of the second.

http://physics.nist.gov/cuu/Units/current.html

The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.

The above definition of the meter is the same as at the NIST site.
 
Brian said:
Yes, I meant defined. RussDill mentioned using wavelength, that may be the "out" of the loop was looking for.
In the case above, let's use 300,000 meters per second. How is the meter defined, the second?* Now, I don't want this to come of as me trying to be difficult, but whatever the answer is, I'm going to ask how you define that unit of measurement.
See what I mean, to me it looks like a loop.

*a second isn't defined as the length of time a particle of light takes to travel 300,000 meters, is it?
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Yes, units of measurement are largely a loop. If you think about what they are describing, it makes sense. There are some units of measurement that are directly scaled to a physical property, ie, 1 electron volt, but it is really not fundamentaly different that measuring that quantity in any other unit.

iirc, you can also derive c using maxwells equations.
 
RussDill said:
Yes, units of measurement are largely a loop. If you think about what they are describing, it makes sense.
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Given the calibration of the second in terms of cesium and the meter in terms of light travel, I'm not sure where you see the loop coming in. That's two independent calibration standards.

iirc, you can also derive c using maxwells equations.

Sort of. It's derived in terms of two other physical constants, which depend on your system of units and which ultimately (I think) come back to how you define the meter and the second.
 
RussDill said:
Yes, units of measurement are largely a loop. If you think about what they are describing, it makes sense. There are some units of measurement that are directly scaled to a physical property, ie, 1 electron volt, but it is really not fundamentaly different that measuring that quantity in any other unit.

iirc, you can also derive c using maxwells equations.
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Let me spew out what got me into this train of thought. I was talking to a friend about how if you had a ruler that was to some degree inaccurate and used it to make a yardstick, the yardstick would be 3 times as inaccurate as the ruler. If that assumption is flawed we could just stop right here.
Now, if all measurements of speed and distance were dependent on each other would they: 1. become more and more inaccurate? or 2. Rigidly "lock up" and become static?
I hope the term "lock up" isn't too vauge. By static I mean "hit a certain value and stay there".
I'm not trying to prove anything here, I don't even have a theory of any kind. I was just thinking about yardsticks.
 
Well you don't want to measure anything accurately using a yardsticks that possess different errors thats statistics I.E. +- %3 over scale, If however they all posses the same degree of error the outcome of measurement could still be accurate.

Putting together a set of yardsticks that consist of a foot a meter and a mile would not give you any worthwhile measurement UNLESS there is a constant involved*. c IS that constant. We can measure arbitrary distances of differing scales and state that D= c*t. We start the stopwatch when the light is emitted and stop at the end of the arbitrary length. By using the T that elapsed between the two events ( start and stop) We can tell the distance traveled. The differences in the yardsticks is irrelevant.

Edit: Actually a foot a meter and a mile are known quantities so You could gleen D from their sum. I was just trying to illustrate a point.
 
Brian said:
Let me spew out what got me into this train of thought. I was talking to a friend about how if you had a ruler that was to some degree inaccurate and used it to make a yardstick, the yardstick would be 3 times as inaccurate as the ruler. If that assumption is flawed we could just stop right here.
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The problem here is what you mean by accuracy. Do you mean fractional, or absolute? While in absolute terms, yes, you would expect a yard stick to reflect three times the error as the footlong ruler you made it from (assuming no additional error in the process, which might not be true), in fractional terms, the error remains the same. And this distinction is important, because there are sometimes ways you can scale a measurement up or down so that relative errors become more relevant than absolute errors (you can bisect your yardstick, for example, to make an 18-inch ruler, which will have the same fractional error as your yardstick). There are cases where the scaling process can involve many orders of magnitude (for example, diffraction gratings that separate out wavelengths on the nanometer scale to differences observable on the millimeter scale). The trick in all of this, of course, is that these translation processes (going from a footlong ruler to a yardstick, etc) are all possible sources of additional error, and if you can't keep a lid on those (or at least undertand and keep track of how big they are), then you're up a creek.
 
Pop quiz:

You travel from point A to point B at 100 kph,
you travel back from point B to point A at 120 kph.

What was your average speed? (it's not what you think)
 
Rocky said:
Pop quiz:

You travel from point A to point B at 100 kph,
you travel back from point B to point A at 120 kph.

What was your average speed? (it's not what you think)
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A<->B=d
t1=d/100kph
t2=d/120kph
totalt=t1 + t2
totald=2*d
avg=totald/totalt

avg=2*d/(t1 + t2)
avg=(2*d(d/100kph + d/120kph)) * (1/d)/(1/d)
avg=2/(1/100kph + 1/120kph)
avg=~109.1kph
 
RussDill said:
A<->B=d
t1=d/100kph
t2=d/120kph
totalt=t1 + t2
totald=2*d
avg=totald/totalt

avg=2*d/(t1 + t2)
avg=(2*d(d/100kph + d/120kph)) * (1/d)/(1/d)
avg=2/(1/100kph + 1/120kph)
avg=~109.1kph
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What can I say except...

VERY well done!

I own a timing company that certifies world records. I have spent years trying explain this to engineers and scientists (I just tell lay people to trust me).

P.S. Exciting video of me crashing at 195 mph can be seen at my web site: www.ricevigeantracing.com
 
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