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Speed and Distance...

Brian

Graduate Poster
B
Joined
Jul 27, 2001
Messages
1,776
So, we measure a distance by how far an object will travel in a certain amount of time. And we measure a speed by how long it takes an object to travel a certain distance. But, we measure that distance by how far an object travels in a certain amount of time...

I think I'm missing something here, I don't know what, but something...
 
"we measure that distance by how far an object travels in a certain amount of time...

I think I'm missing something here, I don't know what, but something..."

You're not missing something, you're putting something in that doesn't belong, specifically the part about "an object travels in a certain amount of time..."
 
Sobriety? ;)

Distance can be measured by other methods besides the elapsed travel time and speed of an object. By reference to another unit of length, for example (which may be standard or arbitrary).
 
My recollection is that time is defined in terms of a frequency of the cesium atom (could be wrong about that), and speed is defined in terms of c. Distance is then derived from that. c is taken to be a fundamental unit, not derived from anything else.
 
Zep said:
Sobriety? ;)

Distance can be measured by other methods besides the elapsed travel time and speed of an object. By reference to another unit of length, for example (which may be standard or arbitrary).
Click to expand...
What methods?
How was that unit of length determined?
Yup. Drunk. :)
 
Art Vandelay said:
My recollection is that time is defined in terms of a frequency of the cesium atom (could be wrong about that), and speed is defined in terms of c. Distance is then derived from that. c is taken to be a fundamental unit, not derived from anything else.
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Distance is measured in terms of multiples or fractions of a metal (I think platinum or palladium) bar kept in Paris. It has nothing to do with the speed of light. Speed in general has nothing to do with the speed of light.

Nonetheless, I would be remiss if I did not mention that you are looking very lovely today.
 
Art Vandelay said:
My recollection is that time is defined in terms of a frequency of the cesium atom (could be wrong about that), and speed is defined in terms of c. Distance is then derived from that. c is taken to be a fundamental unit, not derived from anything else.
Click to expand...
How is c determined? By distance or speed?
 
TeaBag420 said:
"we measure that distance by how far an object travels in a certain amount of time...

I think I'm missing something here, I don't know what, but something..."

You're not missing something, you're putting something in that doesn't belong, specifically the part about "an object travels in a certain amount of time..."
Click to expand...
Honestly, I don't get it.
 
TeaBag420 said:
Distance is measured in terms of multiples or fractions of a metal (I think platinum or palladium) bar kept in Paris. It has nothing to do with the speed of light. Speed in general has nothing to do with the speed of light.

Nonetheless, I would be remiss if I did not mention that you are looking very lovely today.
Click to expand...

BS. The meter is defined as "the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second" since 1984. In 1960 the meter was defined in terms of the wavelength in vacuum of the radiation corresponding to a transition between specified energy levels of the krypton 86 atom. Before that is was the palladium/iridium alloy bar. You're only 44 years behind.

BIPM
 
Brian said:
How is c determined? By distance or speed?
Click to expand...

Distance can also be determined by wavelength, not just speed. Given the particular energy of a photon, what is its wavelength type of thing.

Distance and speed are clearly interrelated, one is defined as meters, the other meters/second. So they are just two sides of the same coin. I think the important thing here is that you need the factor of time to define distance, which seems like no accident to me.
 
Donks said:
In 1960 the meter was defined in terms of the wavelength in vacuum of the radiation corresponding to a transition between specified energy levels of the krypton 86 atom.
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I'm pretty sure it was Cesium. I learned it in college, even knowing exactly how many wavelengths it was - nine billion and change. I wasn't aware that the meter had been redefined in terms of c.
 
Oops, sorry - it *was* Krypton 86. The Cesium in my memory was for the definition of the second, which is still how a second is defined.
 
Distance initally had nothing to do with anything moving. It had more to do with common, relatively similar in size, objects early humans had... namely, body parts.

Even today we use "hands" and "feet" as measurements, although you probably only use "hands" as a unit of measurement if you spend time around horses.

Distance is the size of something, and is independent of any measurement of time.
 
Beleth said:
Distance initally had nothing to do with anything moving. It had more to do with common, relatively similar in size, objects early humans had... namely, body parts.

Even today we use "hands" and "feet" as measurements, although you probably only use "hands" as a unit of measurement if you spend time around horses.

Distance is the size of something, and is independent of any measurement of time.
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Some backwards countries still use "thumbs", although they prefer to call them "inches."
 
Art Vandelay said:
c is taken as basic unit. So what do you mean by "determined"?
Click to expand...

I think he means defined.
c= distance over time.
The distance can be of any arbitrary unit I.E. meters,feet, miles, A/U. It doesn't matter, the answer will be equivalent across D.

Since speed is defined as distance over time , you cannot measure c in relation to speed. c IS a speed. It is the speed of light in a vacuum.

The "speed" of c is 186,000 miles per second or 300,000 meters per second. The only thing that has changed is the units we use to describe distance. You could lay out Ax handles in a line and determine how long in Ax handles it took for light to travel the distance from the first ax handle to the last .

Your car probably has a speedometer that has two scale yes? One for MPH one for KPH, the distance traveled remains the same , the names have been changed to protect the innocent ( sorry '60's TV reference)
 
TillEulenspiegel said:
I think he means defined.
c= distance over time.
The distance can be of any arbitrary unit I.E. meters,feet, miles, A/U. It doesn't matter, the answer will be equivalent across D.

Since speed is defined as distance over time , you cannot measure c in relation to speed. c IS a speed. It is the speed of light in a vacuum.

The "speed" of c is 186,000 miles per second or 300,000 meters per second. The only thing that has changed is the units we use to describe distance. You could lay out Ax handles in a line and determine how long in Ax handles it took for light to travel the distance from the first ax handle to the last .

Your car probably has a speedometer that has two scale yes? One for MPH one for KPH, the distance traveled remains the same , the names have been changed to protect the innocent ( sorry '60's TV reference)
Click to expand...
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?
 
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?
Click to expand...

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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