hI-OOPS-CAPS Posted October 27, 2008 Report Share Posted October 27, 2008 I heard the most amazing thing in one of my lectures, and i just had to post it here since ive got no friends to tell.consider a box, and in the middle is a wheel, surrounded by air. now if you give the wheel a turn, it starts spinning. it will eventually slow down and stop.the air around it partially caused it to slow down - air resistance, and in doing so heated up ever so slightly.now when the wheels stopped, why cant the air then cool down and the wheel start spinning again. its perfectably able to with the 1st law thermodynamics (ie its just energy being transferred) but we all know it wont.id imagine theres a large amount of people deeply curious about this, so my next lecture is sometime during this week Quote Link to comment Share on other sites More sharing options...
Dr. Nick Riviera Posted October 27, 2008 Report Share Posted October 27, 2008 You could argue the potential for this to happen, but it could only be proven possible by the first law if it were bound only by the first law.Where does the energy come from to start the wheel spinning again? Quote Link to comment Share on other sites More sharing options...
Walleee Posted October 27, 2008 Report Share Posted October 27, 2008 cos the energy isn't going into making the wheel turn again? surley it jjust dissapates into the box, heating it up a bit, which will then be transferred into the air surrounding the box. Quote Link to comment Share on other sites More sharing options...
monkeyseemonkeydo Posted October 27, 2008 Report Share Posted October 27, 2008 It's got nowt to do with thermodynamics from what I can tell. An object at rest will stay at rest until a force acts upon it to produce a motion. The wheel needs a force input to start it spinning. Warm air is unable to provide this force.The reason the wheel produces heat in the first place is due to friction (air resistance/drag from the air and friction in the bearings). That heat is then lost to the surroundings, becoming apparent as a slight temperature increase in the box, air and wheel assembly itself. That heat can't magically be transformed into kinetic energy in the wheel without a force acting...Edit: Or am I missing something? Quote Link to comment Share on other sites More sharing options...
ben_travis Posted October 27, 2008 Report Share Posted October 27, 2008 Yup i am agreed with dave! There box was stationary originally, and once a force was applied it moved. Now the force is not applied, the resistance in the wind caused the box to stop. Now no force is applied surely the box cannot move. Quote Link to comment Share on other sites More sharing options...
monkeyseemonkeydo Posted October 27, 2008 Report Share Posted October 27, 2008 Disappointingly uninteresting logic scenario I'm afraid... anyone got any more? And don't you dare ask about any aircraft. Quote Link to comment Share on other sites More sharing options...
Haz Posted October 27, 2008 Report Share Posted October 27, 2008 If there is a helicopter, on a turntable, which spins at the same speed as the rotors on the helicopter, will it take off? Quote Link to comment Share on other sites More sharing options...
Elliot Posted October 27, 2008 Report Share Posted October 27, 2008 If there is a helicopter, on a turntable, which spins at the same speed as the rotors on the helicopter, will it take off?rotor and turntable spinning in the same direction? Quote Link to comment Share on other sites More sharing options...
Krisboats Posted October 27, 2008 Report Share Posted October 27, 2008 If there is a helicopter, on a turntable, which spins at the same speed as the rotors on the helicopter, will it take off?Yes, because the tail rotors on the helicopter will keep it stable as the turntable slides underneath it. Tried it with a PiccoZ Unless of course your talking about a real helicopter and not a toy one. In which case it will still take off because the turntable won't be strong enough to hold it and it'll be smashed underneath it. Quote Link to comment Share on other sites More sharing options...
anzo Posted October 27, 2008 Report Share Posted October 27, 2008 Tried it with a PiccoZHaha, don't get out much, no? I can't fly those bloody things to save my life - although they do hurt then they crash into your face (the Derby Gadget Shop Disaster of 2007). Quote Link to comment Share on other sites More sharing options...
Krisboats Posted October 27, 2008 Report Share Posted October 27, 2008 Haha, don't get out much, no? I can't fly those bloody things to save my life - although they do hurt then they crash into your face (the Derby Gadget Shop Disaster of 2007).I don't normally anyway, but this was due to being stuck at my aunts house on boxing day with no booze so me and my cousin were messing about with his new little helicopter.I managed to navigate it all the way down the stairs and through 2 rooms without crashing They're great with a drawing pin stuck in the end because they go 10x faster. Quote Link to comment Share on other sites More sharing options...
JT! Posted October 27, 2008 Report Share Posted October 27, 2008 If there is a helicopter, on a turntable, which spins at the same speed as the rotors on the helicopter, will it take off?Hahahahahahahahahaha!No it wouldn't. Quote Link to comment Share on other sites More sharing options...
Bionic Balls Posted October 27, 2008 Report Share Posted October 27, 2008 as above...can you explain what was so amazing about this lecture?superconductors and the freaky properties of nanoparticles get me interested lately Quote Link to comment Share on other sites More sharing options...
JT! Posted October 27, 2008 Report Share Posted October 27, 2008 as above...can you explain what was so amazing about this lecture?superconductors and the freaky properties of nanoparticles get me interested lately http://en.wikipedia.org/wiki/Bose-Einstein_condensate Quote Link to comment Share on other sites More sharing options...
manuel Posted October 27, 2008 Report Share Posted October 27, 2008 Disappointingly uninteresting logic scenario I'm afraid... anyone got any more? And don't you dare ask about any aircraft.(can lobster dogs fly ?)Consider the lily.... Quote Link to comment Share on other sites More sharing options...
OD404 Posted October 27, 2008 Report Share Posted October 27, 2008 its perfectably able to with the 1st law thermodynamics (ie its just energy being transferred) but we all know it wontAccording to the 1st Law of thermodynamics it can happen. However, according to the 2nd law of thermodynamics it is impossible and that is all the answer you need EDIT: Unless anyone really wants me to write out the 2nd Law... Quote Link to comment Share on other sites More sharing options...
doopdoopbedoop Posted October 27, 2008 Report Share Posted October 27, 2008 EDIT: Unless anyone really wants me to write out the 2nd Law...Oh, go on then... Quote Link to comment Share on other sites More sharing options...
Greetings Posted October 27, 2008 Report Share Posted October 27, 2008 This is slightly related to the topic, I think it's BMW which is working on catching heat from the engine/exhaust and turning it into extra power using a steam engine mounted alongside the combustion engine. This apparently already works very well and significantly increases efficiency. Quote Link to comment Share on other sites More sharing options...
Luke Rainbird Posted October 27, 2008 Report Share Posted October 27, 2008 Perhaps this will clear things up a little: Youtube Video -> ">" type="application/x-shockwave-flash" wmode="transparent" width="425" height="350"> Quote Link to comment Share on other sites More sharing options...
Trials Punk Posted October 27, 2008 Report Share Posted October 27, 2008 Perhaps this will clear things up a little: Youtube Video -> ">" type="application/x-shockwave-flash" wmode="transparent" width="425" height="350"> Quote Link to comment Share on other sites More sharing options...
OD404 Posted October 27, 2008 Report Share Posted October 27, 2008 You asked for it.Ok well most people are aware of the first law of thermodynamics which states that in any process, energy is conserved. For the above example the kinetic energy of the wheel is converted to heat energy contained in the surrounding air.However we also have the second law of thermodynamics which states that these processes of energy transfer have a direction associated with them and as such are not reversible (as seen by the heat energy in the air not converting to kinetic energy in the wheel). The second law also tells us that different forms energy have a quality to them, as well as a quantity (for example, heat energy at a higher temperature is easier to convert than if it was at a lower temperature). This second aspect of the 2nd law is applicable to the above problem as the air will not be at all hot from slowing the wheel down and therefore extracting energy from the air would be a difficult. I could then start talking about entropy which is a measure of energy's quality, but it is the most confusing concept in thermodynamics and I don't think I would be able to explain it well enough for anyone else to actually grasp what I'm babbling on about So to conclude the wheel would not move from the heat in the air because:a: The process is not reversibleb: The quality of the energy in the air is so low that extracting it is impossibleAnd these are both statements associated with the 2nd law of thermodynamics. Quote Link to comment Share on other sites More sharing options...
Fixed Pants™ Posted October 27, 2008 Report Share Posted October 27, 2008 It needs the force to start it again, that is so obvious, even a twat like me could work it out.Does the aeroplane take off? Quote Link to comment Share on other sites More sharing options...
Pashley26 Posted October 27, 2008 Report Share Posted October 27, 2008 Does the aeroplane take off?Yepppp, so does the helicopter.... Quote Link to comment Share on other sites More sharing options...
Fixed Pants™ Posted October 27, 2008 Report Share Posted October 27, 2008 But what about a runway moving in the opposite direction? Quote Link to comment Share on other sites More sharing options...
OD404 Posted October 27, 2008 Report Share Posted October 27, 2008 This is slightly related to the topic, I think it's BMW which is working on catching heat from the engine/exhaust and turning it into extra power using a steam engine mounted alongside the combustion engine. This apparently already works very well and significantly increases efficiency.On a similar note to this (I'm unaware of BMW's work in this field) look up the 6-stroke cycle. There is also a paper written by Harry Ricardo (the god-father of combustion research) where he ran a wwII aircraft engine (say for sake of argument running 2000bhp), then began to inject water into the cylinders whilst reducing the amount of fuel being used. If memory serves, he managed to reduce the amount of fuel being injected by 70% whilst maintaining the 2000bhp output of the engine.Both these methods work by using the heat in the combustion chamber to rapidly heat the water, which vaporises almost instantly causing massive pressure rises inside the cylinder in a very similar fashion to standard combustion schemes. In the case of the 6-stroke cycle, it actually uses residual heat within the cylinder as opposed to replacing fuel with water in the combustion event itself. Quote Link to comment Share on other sites More sharing options...
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