Pashley26 Posted October 27, 2008 Report Share Posted October 27, 2008 (edited) But what about a runway moving in the opposite direction?The wheels just spin twice as quick.... move as fast as the belt is moving added to the forward speeds of the craft... Edited October 27, 2008 by Pashley26 Quote Link to comment Share on other sites More sharing options...
Fixed Pants™ Posted October 27, 2008 Report Share Posted October 27, 2008 Like a flash boiler/ i know someone who made a potato gun powered by steam, pretty good.The wheels just spin twice as quick.... move as fast as the belt is moving added to the forward speeds of the craft...Stop it, i'm trying to cause a massive, interesting debate Quote Link to comment Share on other sites More sharing options...
OD404 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.It's not just a force that's required. What it actually needs is a work transfer (work being a form of energy, bringing us back to the 2nd law again ). Work is defined as Force X Distance travelled in the direction of the forceIf you put your finger on the wheel and pushed towards the its centre the wheel wouldn't move, even though you applied a force. However if you applied a force in a direction in which the wheel could move, then as the wheel would move energy would be transferred to it.There is a way to get the wheel to move using heat, but that would require more heat than would be in the air due to this little situation Quote Link to comment Share on other sites More sharing options...
Fixed Pants™ Posted October 27, 2008 Report Share Posted October 27, 2008 The force... ie a force that would make it spin Quote Link to comment Share on other sites More sharing options...
OD404 Posted October 27, 2008 Report Share Posted October 27, 2008 The force... ie a force that would make it spin Heat isn't a force though... Quote Link to comment Share on other sites More sharing options...
Fixed Pants™ Posted October 27, 2008 Report Share Posted October 27, 2008 Didn't say it needed the heat, said it needed the force that would make it spin to spin. And as heat isn't a force, the heat wouldn't make it spin again. Quote Link to comment Share on other sites More sharing options...
OD404 Posted October 27, 2008 Report Share Posted October 27, 2008 My argument is that you could use heat to make it spin again, but more heat than is in the air from slowing down the wheel which is why the original scenario would never happen (as you rightly point out).EDIT: it's more a case of explanations and wires being crossed than either of us actually being wrong in this instance Quote Link to comment Share on other sites More sharing options...
george_seamons Posted October 27, 2008 Report Share Posted October 27, 2008 But you can heat the air around the wheel, which would eventually start the wheel turning. So it would turn again.EDIT: Seems Steve said exactly the same thing alot quicker than me Quote Link to comment Share on other sites More sharing options...
OD404 Posted October 27, 2008 Report Share Posted October 27, 2008 But you can heat the air around the wheel, which would eventually start the wheel turning. So it would turn again.EDIT: Seems Steve said exactly the same thing alot quicker than me Not by just heating the air there needs to be an interim step in the process I have in mind. Put it this way, if you spun your bike wheel in your bedroom, and let it come to rest, then set fire to your room, do you think the bike wheel would turn again (assuming it doesn't get damaged in the inferno you created in the name of science)? Quote Link to comment Share on other sites More sharing options...
george_seamons Posted October 27, 2008 Report Share Posted October 27, 2008 Not by just heating the air there needs to be an interim step in the process I have in mind. Put it this way, if you spun your bike wheel in your bedroom, and let it come to rest, then set fire to your room, do you think the bike wheel would turn again (assuming it doesn't get damaged in the inferno you created in the name of science)?Surely it depends how light the wheel is? Going by the same principle as a hot air balloon, surely it would be possible to move a very very lightweight (probably paperweight!) wheel with purely heat? (presuming again, that it didn't catch fire!) Quote Link to comment Share on other sites More sharing options...
Luke Rainbird Posted October 27, 2008 Report Share Posted October 27, 2008 If you could get the wheel in the right orientation, on a frictionless bearing, with the hot air BELOW part of it then it'd be possible as heat rises, however the warmer air would have been at the top of the box when the wheel had stopped so I'd imagine it'd be pretty hard Quote Link to comment Share on other sites More sharing options...
OD404 Posted October 27, 2008 Report Share Posted October 27, 2008 Surely it depends how light the wheel is? Going by the same principle as a hot air balloon, surely it would be possible to move a very very lightweight (probably paperweight!) wheel with purely heat? (presuming again, that it didn't catch fire!)Hot air balloons work on the principal that heat rises. In the case of the wheel, while air at the bottom may want to rise, the air at the top would have to be forced down. The energy required to push the air at the top downwards will be equal and opposite to that created by the air wanting to rise from the bottom. And given that any body experiencing equal and opposite forces will maintain a constant velocity (which for an object at rest is zero) the wheel won't move. Quote Link to comment Share on other sites More sharing options...
george_seamons Posted October 27, 2008 Report Share Posted October 27, 2008 Ok, fair enough. Shows how much I know, haha! Thanks for informing me though, its actually pretty interesting!I was just imagining a water wheel styled wheel, just using hot air instead of water (and the air going bottom to top, not top to bottom)...I was actually basing my theory on being outside the box. Which I guess was wrong. In my scenario, would the same still apply? Quote Link to comment Share on other sites More sharing options...
Haz Posted October 27, 2008 Report Share Posted October 27, 2008 Now this is a party! Quote Link to comment Share on other sites More sharing options...
OD404 Posted October 27, 2008 Report Share Posted October 27, 2008 Ok, fair enough. Shows how much I know, haha! Thanks for informing me though, its actually pretty interesting!I was just imagining a water wheel styled wheel, just using hot air instead of water (and the air going bottom to top, not top to bottom)It could be made to work, but the heat gradients would have to be massive and that's not easy to do. I probably should have said in my previous post that I was assuming the raging inferno was heating the room up uniformly meaning there would be no temperature gradient in the air in the tyre.Either way though we are all talking about putting more energy into the "system" so to speak and I would imagine that when the original poster attends his lecture later this week the topic du jour will be the 2nd law of thermodynamics.And my idea on moving the wheel with heat involved thermal expansion for those wondering what I had in mind (yeah right ) Quote Link to comment Share on other sites More sharing options...
Dr. Nick Riviera Posted October 27, 2008 Report Share Posted October 27, 2008 Pi is exactly 3. Quote Link to comment Share on other sites More sharing options...
OD404 Posted October 27, 2008 Report Share Posted October 27, 2008 So that's where I've been going wrong all these years Quote Link to comment Share on other sites More sharing options...
Walleee Posted October 31, 2008 Report Share Posted October 31, 2008 This is such a kick ass thread we need more of these. Quote Link to comment Share on other sites More sharing options...
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