EPT
Help on physics question
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Blind
United States2529 Posts
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fanatacist
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HeadBangaa
United States6512 Posts
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Meta
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Dromar
United States2145 Posts
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Reflex
Canada703 Posts
Hohoho. (sorry, I just came from a tree pun thread) | ||
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BluzMan
Russian Federation4235 Posts
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Maenander
Germany4926 Posts
Bernoullis formula says that the sum of dynamic pressure, pressure due to elevation and static pressure is constant. Now we can safely neglect pressure due to elevation. Dynamic pressure comes from the velocity of the air. Static pressure is what we measure. Now static pressure is measured as force per area. If you measure static pressure, you always need to have an area to measure the static pressure relative to that area. In this case I guess the area is defined by the window, and thus the air outside is moving compared to that area. Which means static pressure from outside is lower. The woman is right in a sense. If you would take an area not moving with the car, things would be different. | ||
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Asta
Germany3491 Posts
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Maenander
Germany4926 Posts
 Bernoulli´s law is dependent on velocities. That alone tells you that it cannot be defined independently of the inertial frame of reference. The physical effects must be the same regardless of your inertial frame. Bernoulli came up with his law while observing pressure in tubes. If the liquid moves faster, the pressure you measure is lower. Now if you look at the restframe of the liquid, it doesn´t move at all. A simple Galileo transformation makes Bernoulli´s Principle go absurd? No. Because the restframe of the walls is what matters and that is where you measure the pressure. So in the case of a window, the restframe of the window is where you measure pressure differences. It doesn´t matter if the car is driving at 50 kilometers per hour or if the wind is streaming around a parking car with the same velocity. The effect is the same: There is a pressure difference, the outside pressure is lower. If you put glass in the window, you have the same net force from the inside on the glass in both cases. That is what I think is meant by pressure differences are not relative. edit: now that I think about it, the whole point behind Bernoulli´s principle IS that pressure is relative. A moving liquid has lower static pressure than one at rest. So much for pressure as a velocity-independent concept  | ||
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betaben
681 Posts
http://en.wikipedia.org/wiki/Bernoulli's_equation stresses it must be a streamline, i.e. not an enclosed space like a car. so a short answer (but not necessarily complete) is that the equation is only applicable to the air moving wrt alice which will seem like a streamline to her, but the equation is not relevant to Bob, since the car is definitely not a stream line. a corollary to this is that alice will experience a lowering of pressure from the moving air until the air pressure in the car = the low pressure from the moving stream. So in the end, the pressure in the car will be lower than the (stationary) pressure outside the car, (which would be as Bob sees it), but not because of Bernoulli\'s equ. acting as veiwed by bob. | ||
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Jim
Sweden1965 Posts
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Klockan3
Sweden2866 Posts
"Since she is driving with the window open the pressure must be the same inside as outside the car just beacuse the air would instantly even it out if it weren't due to the laws of entropy" I would guess that the question is just to trick you into using formulas you are currently using into a context were it isn't required but instead clearly wrong and a good physicists should notice that instantly. Ofcourse we could ask the question what would happen if it were a closed space, and then the answer would be that the pressure should be larger from the inside since the speed must be relative to the contact surface or the principle would become absurd even in the simplest forms due to the fact that relatively speaking the fluid can be moving at near light speed. However in conditions such as were an enclosed object is traveling through air is very hypothetical, and most likely you wouldnt notice anything due to the turbulent air movement around an objext moving that fast overshading the effects of lowered pressure due to speed difference. | ||
micronesia
United States24759 Posts
Physics Reality Check: Those of you who are claiming to be knowledgeable on this topic, please tell me if you agree or disagree with me on these so we can get on to the same page: 1) When you hold a straw up to your mouth, the pressure of the air is higher in the straw, than outside the straw. 2) An airplane wing is designed to create a large lift force, at the expense of a drag force. This effect is mostly caused by influences other than the Bernoulli Principle. 3) When you point a hairdryer up, and place a ping pong ball in the stream, it hovers. This is not explained simply by using a Bernoulli explanation. | ||
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betaben
681 Posts
http://en.wikipedia.org/wiki/Carburetor "The carburetor works on Bernoulli's principle: the faster air moves, the lower its pressure. The throttle (accelerator) linkage does not directly control the flow of liquid fuel. Instead, it actuates carburettor mechanisms which meter the flow of air being sucked into the engine. The speed of this flow, and therefore its pressure, determines the amount of fuel drawn into the airstream." thus, yes, the pressure in the car is the same as that of the exterior measured by Alice, but this is lower than a the pressure of the exterior measured by an observer who is moving at the same speed as the exterior air. | ||
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Klockan3
Sweden2866 Posts
On November 17 2007 10:20 betaben wrote: Klockan3, the moving air definitely causes a lower pressure; this is how a carburetor works: http://en.wikipedia.org/wiki/Carburetor "The carburetor works on Bernoulli's principle: the faster air moves, the lower its pressure. The throttle (accelerator) linkage does not directly control the flow of liquid fuel. Instead, it actuates carburettor mechanisms which meter the flow of air being sucked into the engine. The speed of this flow, and therefore its pressure, determines the amount of fuel drawn into the airstream." thus, yes, the pressure in the car is the same as that of the exterior measured by Alice, but this is lower than a the pressure of the exterior measured by an observer who is moving at the same speed as the exterior air. No, you dont get it, as long as the air can move freely and the car doesnt change its speed extremely quickly the air will even out any pressure differences caused by any laws at all near instantly. You can't have 2 air spaces in direct connection to each other were one of them have a constant higher pressure than the other, they will just simply move over to the other side untill the pressure is equal. If you didn't understand this, then ask yourself this question, since the pressure is higher from the inside than the outside then shouldn't air constantly be leaving the car due to the pressure differences? And then, what happens when you remove the number of molecules in a gas, hmmm, maybe the pressure drops? And pressure isn't relative, the relativistic laws of physics only matters in situations were laws such as bernouilli and friction doesn't work any longer and were things such as pressure is just as abstract as the relativistic laws are to us. 3) When you point a hairdryer up, and place a ping pong ball in the stream, it hovers. This is not explained simply by using a Bernoulli explanation. Haha, if you applied it on that case youd guess that the ball would get sucked towards the fan beacuse the air speed is higher the closer you get to it! Nice example of how useless the principle can be in situations were the gas/fluid is'nt moving in a very restricted area. | ||
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Maenander
Germany4926 Posts
Klockan3: a concept being relative to an initial frame of reference does not mean it is connected to relativistic physics lol. Yes pressure differences on a small scale will vanish in short time, but I don´t think that was the solution. Otherwise the question of how the Bernoulli-principle can still be valid while velocities are different for observers in different restframes is meaningless ... Also if the pressure difference is evened out and the Bernoulli-principle would work, the static pressure you´d measure in the car would still be different from the static pressure you´d measure in Bobs position, as Benaben said correctly. | ||
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