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Post by katsuro on Jul 7, 2010 19:16:19 GMT -5
Basically a place for people to ask all those questions you've always wanted to know the answers to but you've been afraid to ask because you suspect doing so would make look like an idiot. And hopefully people will give the right answers. I'll start with one that's been bugging me since an episode of Mythbusters that proved a lorry full of birds (or toy helicopters) weighs the same when the birds (or choppers) are all in flight as when they are on the floor of the lorry:
If a helicopter produces downforce greater than it's own weight - essentially pushing down more air than the chopper weighs - when it is airborne, how come when it hovers close above people (like when they are rescuiing soemone and they have to get really close) that person isn't crushed to death? What have I misunderstood?
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letipex
Full Member
The true ouroboros
Posts: 197
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Post by letipex on Jul 8, 2010 11:19:05 GMT -5
Okay, I'm going to try, but... [disclaimer]I don't usually try my hand at explaining technical things in english, since my mastery of the language doesn't seem to extend to the point that I can deliver a simple and understandable explanation of physical phenomenon... That said...[/disclaimer]
The blades from the helicopter create additional pressure under it, that serves to maintain the helicopter airbone. In a closed space, such as a box, the total weight doesn't change, since the pressure from the airbone copter is the same as its weight. However, when you start putting your flying rotary machine (^^) in an open space, you have to take into account the fact that the pressurised air under the blade doesn't like to stay pressurised. So it flows out at great speed, creating a wind that we feel when it hovers above you.
Since when you add speed to a fluid, you diminish its pressure (that's bernoulli principle)... voila! You can observe this phenomenon when you blow between two sheets of paper held close together. You would think that they'd be pushed appart from one another, but you can see them grow closer ans closer until they touch. You added velocity to the fluid, thereby diminishing the pressure. Since the pressure was diminished, you effectively "pushed" the sheets together. The wind of a copter is just that... A way for the air to let go of the additional pressure that you created in order for the helicopter to fly. Plus, this wind usually spreads a lot, wich means that the strenght of the additionnal speed and pressure diminishes that much faster in an open space.
I hope this was adequate and understandable.
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Post by katsuro on Jul 8, 2010 13:13:16 GMT -5
I think I mostly understood the general idea. It's probably simple enough, but I grow exponentially stupider with each passing day of my life lol.
So I now have another question - if a helicopter was hovering just above you in a box (why the hell you and a full size helicopter would be in a box I don't know lol) would you then be crushed?
By the way, if english is not your first language then damn, nice work!
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Post by Distind on Jul 8, 2010 20:26:34 GMT -5
So I now have another question - if a helicopter was hovering just above you in a box (why the hell you and a full size helicopter would be in a box I don't know lol) would you then be crushed? Stupid and basic answer, but probably not. The weight of the helicopter is displaced over the full area under the rotors, which may not seem like much at first, but let's take the apache: 48 foot rotor diameter. Area of a disk is Pi Radius Squared, giving us about 1800 square feet to spread the weight over. Loaded weight is around 18,000 lbs. That's about 10lbs of pressure on a square foot. So no, it's not all that likely. At least if it's an attack air craft. Cargo haulers may weigh in at more but I don't know the name of any off the top of my head.
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Post by katsuro on Jul 9, 2010 6:48:17 GMT -5
Good point Distind. It should be obvious, but I don't think my brain works the same as everyone else here. I'm not convinced it works full stop lol.
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Cymraes
Junior Member
Dim marciau ffordd!
Posts: 63
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Post by Cymraes on Jul 20, 2010 5:48:03 GMT -5
I have a couple of weird questions - things that have kind of bugged me for a long time.
Q1. When you look in a mirror, why is it that left and right appear to be swapped over, but not up and down?
Q2. (More philosophical than actual, but still...) If you record an hour of silence, then play it back in a noisy room at full volume, why won't it drown out the noise?
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Post by Distind on Jul 20, 2010 5:57:28 GMT -5
I have a couple of weird questions - things that have kind of bugged me for a long time. Q1. When you look in a mirror, why is it that left and right appear to be swapped over, but not up and down? This one is fairly simple. Light's a wave, light reflected off or generated by an object is visible to those looking at the object. So it pretty much goes, light bounces off of you, hits the mirror, and returns to you. Effectively giving you the perspective of someone who is looking at you from where ever you are in relation to the mirror. In most cases that's standing directly in front of. Setup a few mirrors and you can manage all kinds of crap. A bit similar to the last one actually, with a few exceptions silence is the absense of any sound, so turning it up doesn't accomplish anything. Think of turning up the volume like multiplying zero, no matter how far you do it, you've still got zero. Though if it did I'd have silence blaring from my laptop all day at work. And yes, my answers are probably all going to be somewhat watered down versions of the full truth, if anyone wants to take the full time to explain them feel free.
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Post by Mlle Antéchrist on Jul 20, 2010 11:22:36 GMT -5
When an unstable black hole collapses, where does the matter/energy go? Is the burst of Hawking radiation the matter in the singularity converted to energy form?
And how can a black hole give off radiation while it's 'feeding', if even photons are subject to its gravity?
(theoretically, obviously)
I understand most other aspects of how black holes form and behave, but this is one part that I'm unsure about.
Also, is it hypothetically feasible that the big bang was the result of a 'white hole', the theoretical inverse of a black hole? Since the expansion of the universe would be in line with how a white hole might behave -- once you pass across the event horizon, you can never return to the singularity/the point in space that you originated from, and would forever be moving away from it. Is this feasible (hypothetically)?
Edit: Also, I realize that space itself is believed to be expanding, as opposed to matter simply moving away from a certain point in space, so I'm wondering if the big bang 'white hole' hypothesis can be rectified with that fact?
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Post by Julian on Jul 20, 2010 13:29:54 GMT -5
interesting thread... With the helicopter one, it's simple fluid dynamics. (Just don't try and do the math) The force is not transferrable directly downwards for 1, and 2 and more importantly, a helicopter rotor acts more like a plane wing than say a propellor or a fan. The lift is provided by lower air pressure above the helicopter being created by the path of the rotor, so the air is pushing the helicopter up. And as mentioned earlier, the equal and opposite effect of newton's 3rd law is negated by fluid dynamics.
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Post by Julian on Jul 20, 2010 14:50:27 GMT -5
I have a couple of weird questions - things that have kind of bugged me for a long time. Q1. When you look in a mirror, why is it that left and right appear to be swapped over, but not up and down? This is a bit of a complex question actually depending on how far you want to take it... Let's start by assuming it's a clear flat mirror with a good reflectance coefficient. Short answer is they don't actually swap over, left remains left, but your brain sees someone facing you, and realises that your left is their right in the general 3D modelling your brain and eyes constantly do. longer. OK, so you're familiar with the angle of reflection = the angle of incidence and so forth I take it, and know that it sort of works like this. (go the super cruddy ASCII art). This might be an aerial view, but is actually free of context and equally could be from the side and that's if your splitting it orthogonally. (sorry) Basically, when viewed from any perpendicular direction at all, this happens in the other two dimensions. \...//\/\ ..ô.. The thing is, left/right is not a true reversal (mirror writing for instance). If you stood one metre in front of a mirror and also had a mole on you're right cheek, you would perceive 'someone' 2 metres away from you with a mole on what you perceive to be their left cheek. This is the way your brain faithfully interprets the light - if your eye saw something that reflected off a spot in the mirror 1 foot down and six inches to your left a bit over a metre away (not doing the 3D trig), you would see it in the mirror where it appeared to come from which is 2 feet down and a foot to your left, a bit over two metres away. Down stays down, and left stays left, it just gets a little confusing to the brain (which is still hardwired to caveman or earlier) when the spot you see happens to be a person who happens to be facing you and your brain starts thinking real world stuff about this friend/foe weird fella staring at you, and why the fuck is he drooling... Q2. (More philosophical than actual, but still...) If you record an hour of silence, then play it back in a noisy room at full volume, why won't it drown out the noise? You know about sounds waves, and waves in general I take it? If you think about .wav files for instance or something like that, the sounds waves of the drum, over lap with the guitar which overlaps with the cheesy lyrics etc, and all of these positive waves add together to form one composite shape, which also has the white noise or background noise in it too - which is why recording studios were designed to take great pains to cancel it out if that's their aim and why if they're laying the vocal track down the singer has the music in the track in earphones to stop it being laid down twice. So silence is just unidentifiable white noise. If you cranked it up, you'd just have louder white noise. Noise cancellation earphones work to cancel out relatively constant noises *in combination with a louder one*. They know what's coming out of the mp3 track, and they know what just happened in the immediate surroundings - normally these would add together to get the composite wave you'd hear, but the headphone software subtracts the white noise from the composite wave giving you something that should much more accurately approximate the pure sounds from the mp3 file, with all the other crap filtered out of the sound waves. google noise cancellation headphones or something if you're interested. I daresay "How it works" (can't recall if it's How stuff works) or someone has done something with pretty pictures. (Just had an insanely inane annoying conversation for about 20 minutes and I can't be assed googling it.)
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Post by Napoleon the Clown on Jul 20, 2010 17:12:19 GMT -5
When an unstable black hole collapses, where does the matter/energy go? Is the burst of Hawking radiation the matter in the singularity converted to energy form? Well, when a supergiant goes supernova, it blows away a large portion of its mass. When it collapses into a black hole, it will still have hundreds or thousands of solar masses left. All the matter and energy that's within the black hole's event horizon gets pulled in to a singularity. A singularity being an object that has mass, but no volume. Try not to think too hard on this one... It can be a tad confusing without a decent amount of understanding of the structure of matter. Black holes emit x-rays as they feed because matter being pulled in forms an accretion disk. Because it's being pulled in at such phenomenal speeds, the friction of it rubbing together emits electromagnetic radiation, primarily x-rays. Those that originate outside the event horizon will be able to get away. Remember, it's not until you hit the event horizon that the escape velocity reaches and then exceeds c. [/quote]Also, is it hypothetically feasible that the big bang was the result of a 'white hole', the theoretical inverse of a black hole? Since the expansion of the universe would be in line with how a white hole might behave -- once you pass across the event horizon, you can never return to the singularity/the point in space that you originated from, and would forever be moving away from it. Is this feasible (hypothetically)?[/quote] Very few credible physicists take the white hole concept seriously, as the current models for black holes explain what happens to the matter and energy that get drawn in. Which basically is "It becomes one with the singularity, now and forever." A white hole that simply spewed matter and energy endlessly would violate (very thoroughly, I might add) quite a few laws of physics. There's really no need to rectify a theory with crack hypotheses, to be honest...
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Post by Mlle Antéchrist on Jul 21, 2010 1:47:26 GMT -5
Well, when a supergiant goes supernova, it blows away a large portion of its mass. When it collapses into a black hole, it will still have hundreds or thousands of solar masses left. All the matter and energy that's within the black hole's event horizon gets pulled in to a singularity. A singularity being an object that has mass, but no volume. Try not to think too hard on this one... It can be a tad confusing without a decent amount of understanding of the structure of matter. I should have phrased the question more clearly. 'Collapsing' was the wrong word to use. I understand how black holes form and the concept of a singularity (as best as someone who isn't a physicist can, anyway), but I'm wondering about the theories that say that a black hole has a finite lifespan and will eventually evaporate, for want of a better term. Perhaps these theories have fallen out of favour, but if they haven't... when the black hole "dies", so to speak, where does the mass go? Since matter and energy cannot be created or destroyed, one would assume that the singularity couldn't simply cease to exist. Is the burst of radiation that Hawking theorizes would be given off by this 'dying' black hole made up of everything drawn into it, with all matter converted in energy form? Ah, that makes perfect sense. I didn't realize that it originated outside of the event horizon. I was spending too much time thinking about what goes on past the event horizon to stop and consider that this process might not be occurring within its boundaries. Again, that makes sense. I've been witnessing too many people taking the white hole hypothesis seriously, so I made the mistake of thinking that it had some scientific validity -- not necessarily as the "other side" of a black hole, since that would be impossible, but as something completely different all together. I was actually going to ask how a white hole could even be possible based on known physics, but you answered that question already. Fair enough. Thanks for the answers.
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Post by Julian on Jul 21, 2010 4:01:43 GMT -5
When an unstable black hole collapses, where does the matter/energy go? Is the burst of Hawking radiation the matter in the singularity converted to energy form? And how can a black hole give off radiation while it's 'feeding', if even photons are subject to its gravity? Didn't have time to get to this yesterday. I thought Hawking radiation was a feature of all black holes, and relatively constant given the properties of a black hole and was the reason why they get smaller if the outflow of radiation is greater than the influx. Hawking radiation is created via quantuum fluctuations at the event horizon, so a small amount of the particles/gamma rays created are 'ejected' in such a way that they do not feed back in due to being outside the inexorable gravitational pull and travelling away. For question 1, I think so. (I personally) don't really know enough about what goes on inside the black hole to comment (not sure if I've forgotten or not, it only would've been something I've idly read). I think the burst at the end is firstly due to the event horizon as above moving inwards as more stuff escapes, so even more of the particles escape, at a faster and faster rate, and less are likely to be reabsorbed, followed then (I think) by the fact that gravity suddenly becomes too weak to hold the remaining matter in such a tightly bound state and it rapidly expands out. You should probably google this.
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Post by katsuro on Jul 22, 2010 9:35:38 GMT -5
This one is fairly simple. Light's a wave... I thought light was actually particles called photons? I'd also like to say this thread has quickly turned into stuff that makes my brain turn to liquid when I try to understand some of the things being dicsussed lol. I now feel like the person with the lowest IQ on the forum
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Post by rookie on Jul 22, 2010 10:52:50 GMT -5
I'd also like to say this thread has quickly turned into stuff that makes my brain turn to liquid when I try to understand some of the things being dicsussed lol. I now feel like the person with the lowest IQ on the forum I'm afraid I have to challenge you to that title.
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