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post #16 of 244
7/18/10 at 11:22pm
Quote:
Originally Posted by why View Post
Is that relevant? If so, I'm interested in answers for both possibilities.

If travelling towards, the man would crash into them at the same time as his broadcast is recieved.

If travelling away from, his broadcast would be redshifted to zero energy, so his broadcast wouldn't be a broadcast at all.
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post #17 of 244
7/18/10 at 11:32pm
Quote:
Originally Posted by tagutcow View Post
If travelling towards, the man would crash into them at the same time as his broadcast is recieved.
That is unfortunate for the man but irrelevant to the audience and their reception of the broadcast.
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If travelling away from, his broadcast would be redshifted to zero energy, so his broadcast wouldn't be a broadcast at all.
Of course it would be.
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post #18 of 244
7/18/10 at 11:40pm
Quote:
Originally Posted by why View Post
That is unfortunate for the man but irrelevant to the audience and their reception of the broadcast.
Well now that he's there, he might as well recite it in person. At slightly slower than the speed of light, it would sound something like this: (rilly fast) "AndforthesupportofthisDeclarationwithafirmrelianceontheprotectionofdivineProvidencewemutuallypledgetoeachotherourLivesourFortunesandoursacredHonor**CrAaSSssSSHHh** O hai!"
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Of course it would be.
How can you listen to a radio station operating at zero watts?
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post #19 of 244
7/18/10 at 11:47pm
Quote:
Originally Posted by tagutcow View Post
How can you listen to a radio station operating at zero watts?

I don't think power is relevant.
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post #20 of 244
7/18/10 at 11:56pm
Quote:
Originally Posted by why View Post
I don't think power is relevant.

Well a wave with an infinitely long wavelength, which is what you would get from a broadcast travelling away from you at the speed of light, would carry zero energy. Ignoring that fact, if there were still some theoretical way to view this broadcast, it would be a perpetual freeze-frame.

Caveat: this is all coming from a North Carolina School of Science and Math reject. Surely there's someone on here more knowledgable than I on this subject.
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post #21 of 244
7/19/10 at 12:17am
Quote:
Originally Posted by why View Post
If time slows as velocity increases toward c, then if a person theoretically traveled at c couldn't they continually broadcast their actions to an audience that perceives them at the normal rate of time? How would such actions be interpreted by the audience? The broadcaster could do something like, say, recite the Declaration of Independence, but the audience would receive it as if no time had actually past. So the broadcaster is aware of his recital of the Declaration of Independence and it thus would be said to have actually occurred, yet is such an act even perceptible by the audience? And if not, would it actually occur in 'reality'? What if the velocity of the broadcaster was slowed to only near-c? How would the audience then perceive the recital?

If he was traveling at C, he would be frozen (in time, not in space, since he would be moving pretty quickly) as far as any observers were concerned, so while he was reciting the Declaration, the universe would have reached its end. If he were merely moving almost at C, he would be observed to be speaking extremely slowly.
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post #22 of 244
7/19/10 at 6:27am
Quote:
Originally Posted by MetroStyles View Post
I was just gonna start something like this!!

Why does wind make us feel cooler?

Seriously. No I won't google it because where's the romance in that?

But yeah, I mean ok I understand if it's cold, more cold wind makes us colder...but when it's hot, why does wind make us cooler as well? After all, it's just more hot air blowing at us. If anything speed heats up molecules even more, amirite?

One of you smart assholes answer me. Or one of you dumb-but-capable-of-googling idiots.

There are three main modes of heat transfer. Conduction, convection and thermal radiation. The wind is really convection, which is heat transfer via the movement of a fluid (air and sweat in this case).
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post #23 of 244
7/19/10 at 7:55am
Quote:
Originally Posted by why View Post
[A]ssume the broadcaster is traveling only slightly slower than c. How are the broadcasts received and perceived?

The people on Earth see the broadcaster speak very slowly due to time dilation. The broadcaster sees the people on Earth walk about very slowly due to time dilation.

There is no absolute reference frame, only relative velocity matters. Earth sees the broadcaster moving at slightly less than c. The broadcaster sees Earth moving slightly less than c.
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post #24 of 244
7/19/10 at 8:58am
Quote:
Originally Posted by aKula View Post
The people on Earth see the broadcaster speak very slowly due to time dilation. The broadcaster sees the people on Earth walk about very slowly due to time dilation.

There is no absolute reference frame, only relative velocity matters. Earth sees the broadcaster moving at slightly less than c. The broadcaster sees Earth moving slightly less than c.

Or doesn't see :/ eyes don't work that quickly i guess u would have to say 'doesn't perceive', i guess he would see something...
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post #25 of 244
7/19/10 at 9:11am
Quote:
Originally Posted by MetroStyles View Post
I was just gonna start something like this!!

Why does wind make us feel cooler?

Seriously. No I won't google it because where's the romance in that?

But yeah, I mean ok I understand if it's cold, more cold wind makes us colder...but when it's hot, why does wind make us cooler as well? After all, it's just more hot air blowing at us. If anything speed heats up molecules even more, amirite?

One of you smart assholes answer me. Or one of you dumb-but-capable-of-googling idiots.
As others pointed out, evaporation of sweat is what cools you down.

When the temperature of the air is >37°, however, it ceases to cool the body.
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post #26 of 244
7/19/10 at 9:20am
Quote:
Originally Posted by CunningSmeagol View Post
This is right. The wind robs us of an insulating layer.

See above. I don't think these are nearly as influenced by the wind, but rather by relative humidity.

I'm pretty sure the evaporation explanation is correct, given it's really just an extension of the body's natural cooling process. The point of sweating is to cool the body through evaporation; wind just takes it up a notch, so to speak.

Relative humidity is really only a consideration in the efficacy of sweating/evaporation. Higher humidity makes it harder for the moisture on the skin to evaporate, thus compromising the body's ability to cool itself. This is why 90* in, say, Florida (where the humidity is high) is so much more miserable than 90* in Phoenix (where the humidity is relatively low).
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post #27 of 244
7/19/10 at 9:31am
Quote:
Originally Posted by MrG View Post
I'm pretty sure the evaporation explanation is correct, given it's really just an extension of the body's natural cooling process. The point of sweating is to cool the body through evaporation; wind just takes it up a notch, so to speak.
When you are not sweating on a cold day, the wind still cools you down. The body cools itself through evaporation, sure, but also through convection. Wind chill specifically addresses convection. http://science.howstuffworks.com/question70.htm See? Convection. It's like if you were in cold water and a strong current ripped your wetsuit off.
Quote:
Originally Posted by MrG View Post
Relative humidity is really only a consideration in the efficacy of sweating/evaporation. Higher humidity makes it harder for the moisture on the skin to evaporate, thus compromising the body's ability to cool itself. This is why 90* in, say, Florida (where the humidity is high) is so much more miserable than 90* in Phoenix (where the humidity is relatively low).
That's exactly what I was saying. Notice that the posts I responded to were about heat loss due to the evaporation of sweat. The decreased efficacy of sweating due to humidity is what the heat index is all about. Only relevant in the summer, when you are actually sweating to cool yourself.
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post #28 of 244
7/19/10 at 10:05am
Quote:
Originally Posted by why View Post
If time slows as velocity increases toward c, then if a person theoretically traveled at c couldn't they continually broadcast their actions to an audience that perceives them at the normal rate of time? How would such actions be interpreted by the audience? The broadcaster could do something like, say, recite the Declaration of Independence, but the audience would receive it as if no time had actually past. So the broadcaster is aware of his recital of the Declaration of Independence and it thus would be said to have actually occurred, yet is such an act even perceptible by the audience? And if not, would it actually occur in 'reality'? What if the velocity of the broadcaster was slowed to only near-c? How would the audience then perceive the recital?
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Originally Posted by eMacPaul View Post
If he was traveling at C, he would be frozen (in time, not in space, since he would be moving pretty quickly) as far as any observers were concerned, so while he was reciting the Declaration, the universe would have reached its end. If he were merely moving almost at C, he would be observed to be speaking extremely slowly.
I agree with this answer more than the others. Special relativity states that an object's combined speed through time and space is the speed of light. This means that when you divert an objects speed into motion through space, you necessarily divert speed away from its motion through time. Light is ageless, because all of its motion is through space. A person traveling at the speed of light would not only freeze to observers, but also to himself (assuming he could perceive being frozen, which he couldn't because he would have zero concept of time at that speed). He couldn't recite the declaration, because there can be no sequence of events (motion through time) at that speed. If he's moving close to c, then yes, observers see him talking slowly. They see this because they see light barely outpacing him (so his motion through space must be diverted away from his motion through time). He himself does not feel like he is talking slowly, because compared to his perception of the speed of light (670mm mph relative to everything), he is motionless. When he came back, his JLC would show less time elapsed than the observers' watches.
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post #29 of 244
7/19/10 at 10:22am
Quote:
Originally Posted by holymadness View Post
As others pointed out, evaporation of sweat is what cools you down.

When the temperature of the air is >37°, however, it ceases to cool the body.

Really? I still feel that breezes in really hot places are cooling.
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post #30 of 244
7/19/10 at 10:24am
Quote:
Originally Posted by CunningSmeagol View Post
When you are not sweating on a cold day, the wind still cools you down. The body cools itself through evaporation, sure, but also through convection. Wind chill specifically addresses convection.

http://science.howstuffworks.com/question70.htm

See? Convection. It's like if you were in cold water and a strong current ripped your wetsuit off.

Even on a cold day, there is moisture on your skin, albeit far less than on a hot one.

Wind accelerates evaporation, and evaporation cools the body. Is there a convection component to wind chill? Yes. However, that is not the main component of the cooling effect of wind, and it doesn't explain why wind makes you feel cooler in warm temperatures. Metro specifically asked why wind makes us feel cooler, and he noted that this phenomenon occurs both in cold and warmth. The cooling effect of warm wind cannot be explained by convection, as it's simply more warm air being blown against the body. Indeed, convection can actually make you feel warmer, as is the case with a convection heater.

So yes, cold wind will make you feel colder through convection, but that is not the main process at work when it comes to feeling cooler as a result of moving air. The main process is evaporation. Convection only matters when the air that is moving is cool; evaporation makes you feel cooler over a much broader spectrum of temperatures.

From How Stuff Works' article on how fans work (note the relationship between convection and evaporation on a cold day):

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When weatherpeople talk about wind chill on a cold winter day, what they are referring to is how the wind increases convective heat loss (see How Thermoses Work for details on convection). By blowing air around, the fan makes it easier for the air to evaporate sweat from your skin, which is how you eliminate body heat. The more evaporation, the cooler you feel.

Canadian Centre for Occupational Health and Safety

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Evaporation of sweat from the skin cools the body. Evaporation proceeds more quickly and the cooling effect is more pronounced with high wind speeds and low relative humidity.

Quote:
Originally Posted by CunningSmeagol View Post
That's exactly what I was saying. Notice that the posts I responded to were about heat loss due to the evaporation of sweat. The decreased efficacy of sweating due to humidity is what the heat index is all about. Only relevant in the summer, when you are actually sweating to cool yourself.

I don't think it's exactly what you were saying. High humidity makes it harder for sweat to evaporate because it can't exchange moisture as easily with the air.

From the same Canadian write-up:

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In hot and humid workplaces, the cooling of the body due to sweat evaporation is limited by the capacity of the ambient air to accept additional moisture.
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