Severian Posted August 12, 2004 Share Posted August 12, 2004 I was amused by this question I saw recently on an exam paper. Lets see who can answer it. A van is accelerating continuously at 12 ms^-2. In the back there are two baloons: one is filled with He (lighter than air) and attached to the floor by a piece of string; the oher is filled with C02 (heavier than air) and hangs on a peice of string from the ceiling. Describe the behaviour of the ballons as seen from inside the van. Link to comment Share on other sites More sharing options...
Aeschylus Posted August 12, 2004 Share Posted August 12, 2004 It depends on the level, accelrtaed frames of refernce are opften apt to trip people up. But then again I just say: 'd'Alembert!' Link to comment Share on other sites More sharing options...
pulkit Posted August 12, 2004 Share Posted August 12, 2004 accelrtaed frames of refernce are opften apt to trip people up. People with a lack of apptitude in advanced mechanics Link to comment Share on other sites More sharing options...
Aeschylus Posted August 12, 2004 Share Posted August 12, 2004 People with a lack of apptitude in advanced mechanics Obviously it depends on your level, but if you don't know the procedure you can be tripped up easily as (excepting general relativity) the normal laws of physics (i.e. those that appply to inertial frames) don't always apply to accelarted frames. Link to comment Share on other sites More sharing options...
atinymonkey Posted August 12, 2004 Share Posted August 12, 2004 People with a lack of apptitude in advanced mechanics You mean people who don't study advance mechanics. Plus, advanced mechanics doesn't help you describe physics. And aptitude only has the one p, if you going to be elitist spelling is a requirement. Link to comment Share on other sites More sharing options...
pulkit Posted August 12, 2004 Share Posted August 12, 2004 And aptitude only has the one p, if you going to be elitist Not trying to be elitist, if you ever did a course in advanced engineering level mechanics, its as simple as this : If you can't understand accelerated frames of reference or are not at ease with them, you probably will flunk your course. Plus, advanced mechanics doesn't help you describe physics Of course it doesn't, its only a small part of physics. Link to comment Share on other sites More sharing options...
Aeschylus Posted August 12, 2004 Share Posted August 12, 2004 Accelerated frames of reference can still trip up professional physicists, for example there are still some physicsts who believe that special relativity cannot handle accelerated frames of reference. Link to comment Share on other sites More sharing options...
pulkit Posted August 12, 2004 Share Posted August 12, 2004 I am not referring to relativity here (I know nothing much about that) Atleast in cases where bodies are moving about at speed < 0.1 c , they should present little problem. Link to comment Share on other sites More sharing options...
atinymonkey Posted August 12, 2004 Share Posted August 12, 2004 Not trying to be elitist' date=' if you ever did a course in advanced engineering level mechanics, its as simple as this :If you can't understand accelerated frames of reference or are not at ease with them, you probably will flunk your course.[/quote'] Your missing the point, accelerated frames of reference will only tell you what will happen to the balloons in the car if you have an understanding of Archimedes principle & pseudo-forces etc. You have to be able to understand what the balloons are going to do before you extrapolate using accelerated frames of reference, or all you are doing is complicating a guess. Popping in to say 'people who make mistakes with accelerated frames of reference have no apptitude in advanced mechanics' is a little sweeping to say the least. Link to comment Share on other sites More sharing options...
Severian Posted August 12, 2004 Author Share Posted August 12, 2004 So what's the answer then? Link to comment Share on other sites More sharing options...
swansont Posted August 13, 2004 Share Posted August 13, 2004 He balloon moves forward, CO2 balloon moves backward. From Newton's first law, the air in the van doesn't move forward with it until it builds up pressure to give it the required acceleration, so it builds up a little in the back. This exerts a pressure on the balloons and since the He is lighter, it is displaced forward relative to the van. Link to comment Share on other sites More sharing options...
Severian Posted August 13, 2004 Author Share Posted August 13, 2004 Well done. I was expecting people to say they both mover backwards. You have reafirmed my faith in humanity Link to comment Share on other sites More sharing options...
Primarygun Posted August 14, 2004 Share Posted August 14, 2004 Which force makes the hydrogen particles move upward ? It is larger than the weight of itself. Link to comment Share on other sites More sharing options...
Severian Posted August 14, 2004 Author Share Posted August 14, 2004 Which force makes the hydrogen particles move upward ? Gravity (or if you want to be pedantic' date=' it is really gravity which makes the air pressure below the baloon larger than that above and it is the diferential in air pressure which puches the baloon up....) It is larger than the weight of itself. Is it just me, or do a large proportion of the statements made on this site not make any sense whatsoever? (I am not meaning I am being dazzled by science.) Link to comment Share on other sites More sharing options...
Primarygun Posted August 15, 2004 Share Posted August 15, 2004 You make no sense of my prediction. lol Anyway, it's just my "simple" prediction. Link to comment Share on other sites More sharing options...
5614 Posted August 15, 2004 Share Posted August 15, 2004 (this is a sub-comment, it does not answer the original question, but adds more to the thread) you say, "is this an easy question" i say "what is an easy question" an easy question is a question which you know the answer to, or you know the way to work it out, thus i came to the conclusion that this question, will either be easy, if you know how to do it, or totaly impossible, if you have no idea. Link to comment Share on other sites More sharing options...
pulkit Posted August 15, 2004 Share Posted August 15, 2004 An easy question is one you can solve within a reasonable period of time given you have complete knowledge of all the facts/laws required to solve the question. (Atleast thats what I think) Link to comment Share on other sites More sharing options...
5614 Posted August 15, 2004 Share Posted August 15, 2004 thats kinda what i said, if not, its what i meant, so yes, i agree with pulkit Link to comment Share on other sites More sharing options...
Kedas Posted August 15, 2004 Share Posted August 15, 2004 He balloon moves forward, CO2 balloon moves backward. From Newton's first law, the air in the van doesn't move forward with it until it builds up pressure to give it the required acceleration, so it builds up a little in the back. This exerts a pressure on the balloons and since the He is lighter, it is displaced forward relative to the van. I think you forgot a force/mass: so what you say is a balloon filled with air will not move in the car? then what is accelerating the balloon itself to math the speed of the car?? Link to comment Share on other sites More sharing options...
harsh Posted August 15, 2004 Share Posted August 15, 2004 I think it would be great to know the frames of reference used here......but since its not given ill take a try and base the problem on inertia..... the string connecting the ballon is moving and the helimballon is not it will jerk back wards(taking 2D to the back door of the van)..and the hevier ballon will continously collide with the roof of the van since its hevier....and its postion cant be judged.... Link to comment Share on other sites More sharing options...
pulkit Posted August 15, 2004 Share Posted August 15, 2004 so what you say is a balloon filled with air will not move in the car? I don't intend to offer a solution to this question just another thought........ If indeed an air filled balloon (ideally with a massless membrane) moves, that'd mean air will begin to move backward too. This would establish quite a pressure gardient in the car which seems quite an unreasonable thing to occur. So maybe, it should not move. Link to comment Share on other sites More sharing options...
swansont Posted August 16, 2004 Share Posted August 16, 2004 I think you forgot a force/mass:so what you say is a balloon filled with air will not move in the car? then what is accelerating the balloon itself to math the speed of the car?? The air filled balloon will move back relative to the car. Newton's first law. Link to comment Share on other sites More sharing options...
bloodhound Posted August 16, 2004 Share Posted August 16, 2004 I am completely baffled by this question as this is not at all easy for me. I have seen a similar question. instead of the car acceleraing in the st . line. u have a car going in a circle at constant speed. u have a balloon feelled with CO2 (or H2 or He can't remember) tied down to the centre of the cars interior. and the question asked which way does the balloon lean. Link to comment Share on other sites More sharing options...
pulkit Posted August 16, 2004 Share Posted August 16, 2004 The air filled balloon will move back relative to the car. Newton's first law. Would this hold even if given an ideal massless membrane for the balloon ? Link to comment Share on other sites More sharing options...
Guest tyroc Posted August 16, 2004 Share Posted August 16, 2004 This is similar to the principle of centrifugation. The heavier mass will tend to aggregate further away from the direction of force because it is more difficult to 'move' the heavier mass. remember this is not an equilibrium. In the initial conditions of a body of mixtures, the heavier (or denser) constituents will accelerate slower towards the direction of force than the lighter constituents. A larger mass also means a larger resistance to motion-- Newton's 1st and 2nd laws. If the acceleration is uniform intially for all parts of the body initially than there should not be any movement at all. But this is never the case, acceleration is not uniform and the difference is due to the difference in mass. Ever notice why you jerk in the bus/train when ever it stops or starts? Non-uniformity of acceleration! Link to comment Share on other sites More sharing options...
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