CasualKilla Posted March 23, 2015 Share Posted March 23, 2015 Please refer to the spinning skateboard wheel in the video below. Those of you with the patience to watch it to the end will notice the wheel does indeed stop spinning. Why question to you is why does the wheel stop spinning, more specifically how is the angular momentum be conserved if the wheel stops spinning? Saying friction causes the wheel to lose kinetic energy true, but does not explain how momentum is conserved. GL Link to comment Share on other sites More sharing options...
pzkpfw Posted March 23, 2015 Share Posted March 23, 2015 The momentum isn't conserved, because of that friction. The energy of the spinning gets changed into heat and sound. Link to comment Share on other sites More sharing options...
John Cuthber Posted March 23, 2015 Share Posted March 23, 2015 Angular momentum is transferred to and from the rest of the world. It doesn't notice much. Link to comment Share on other sites More sharing options...
CasualKilla Posted March 23, 2015 Author Share Posted March 23, 2015 1 right answer and 1 wrong answer so far, so tense -1 Link to comment Share on other sites More sharing options...
Greg H. Posted March 23, 2015 Share Posted March 23, 2015 Angular momentum is only conserved in a closed system. The wheel isn't a closed system. Link to comment Share on other sites More sharing options...
CasualKilla Posted March 23, 2015 Author Share Posted March 23, 2015 (edited) Angular momentum is only conserved in a closed system. The wheel isn't a closed system. That answer is weak-sauce, you can do better. It depends on the boundary's you choose yourself. Technically the only truly closed system is the entire universe, make some approximations. Edited March 23, 2015 by CasualKilla Link to comment Share on other sites More sharing options...
swansont Posted March 23, 2015 Share Posted March 23, 2015 You also need to conserve angular momentum when you start the wheel turning, if you choose the system to be large enough. When the person slides his/her foot on the wheel, it's conserved, if the person and the earth are part of the system. We don't notice their rotation because the amount of angular momentum is small. Ultimately it's coupled to the earth's rotation, so there's only ~25 orders of magnitude difference in the mass to worry about. Otherwise, it's because there's a torque from friction in the bearings. Link to comment Share on other sites More sharing options...
Greg H. Posted March 23, 2015 Share Posted March 23, 2015 That answer is weak-sauce, you can do better. It depends on the boundary's you choose yourself. Technically the only truly closed system is the entire universe, make some approximations. Was I supposed to be impressing someone? Link to comment Share on other sites More sharing options...
CasualKilla Posted March 23, 2015 Author Share Posted March 23, 2015 (edited) You also need to conserve angular momentum when you start the wheel turning, if you choose the system to be large enough. When the person slides his/her foot on the wheel, it's conserved, if the person and the earth are part of the system. We don't notice their rotation because the amount of angular momentum is small. Ultimately it's coupled to the earth's rotation, so there's only ~25 orders of magnitude difference in the mass to worry about. Otherwise, it's because there's a torque from friction in the bearings. Yes that is 100% right, you create an opposite angular momentum on the earth when you spin it up, then the angular momentum returns to the initial value after the wheel stops spinning. You can also show that a torque is created on the earth by the friction which is what transfers the impulse. Bonus marks for anybody who can give a estimate of the change of angular momentum of the earth while the wheel is spinning. Make some realistic estimates for the wheel mass and angular velocity! Edited March 23, 2015 by CasualKilla Link to comment Share on other sites More sharing options...
John Cuthber Posted March 23, 2015 Share Posted March 23, 2015 Bonus marks for anybody who can give a estimate of the change of angular momentum of the earth while the wheel is spinning. Make some realistic estimates for the wheel mass and angular velocity! Exactly the same magnitude as that of the wheel, but in the opposite direction. But why do you think you are in a position to be awarding marks? Link to comment Share on other sites More sharing options...
CasualKilla Posted March 24, 2015 Author Share Posted March 24, 2015 Exactly the same magnitude as that of the wheel, but in the opposite direction. But why do you think you are in a position to be awarding marks? My thread, my question. Whether you choose to give value to my marks is up to you. Link to comment Share on other sites More sharing options...
Phi for All Posted March 24, 2015 Share Posted March 24, 2015 My thread, my question. Whether you choose to give value to my marks is up to you. You know, it's not the mention of "marks", or the elementary challenges you pose. It's starting threads asking questions you claim to already know the answers to, and then condemning certain responses during the discussion. That's what seems like preachy soapboxing. That's what diminishes the value of your "marks". It's not a good style for productive discussion. Just sayin'. Link to comment Share on other sites More sharing options...
Greg H. Posted March 24, 2015 Share Posted March 24, 2015 My thread, my question. Whether you choose to give value to my marks is up to you. I'll be somewhat blunter than Phi. I don't know if it's intentional, or you think you're being amusing, or what, but - you come off as an arrogant ass. Link to comment Share on other sites More sharing options...
imatfaal Posted March 24, 2015 Share Posted March 24, 2015 Does the earth need to be involved? Two counter-rotating wheels of the same moment of inertia and magnitude of angular velocity carry no net change in angular momentum when viewed as a system Link to comment Share on other sites More sharing options...
CasualKilla Posted March 24, 2015 Author Share Posted March 24, 2015 (edited) Does the earth need to be involved? Two counter-rotating wheels of the same moment of inertia and magnitude of angular velocity carry no net change in angular momentum when viewed as a system Yes I believe so, otherwise where is the this second counter rotating wheel you speak of? If we span 2 wheels in different direction then what you say is true, i though it would be more interesting to consider one wheel and where the angular momentum is going. It is something I considered yesterday and when it finally clicked spinning the wheel also spins the earth, it was a really interesting realization. I just wanted to share that feeling! Exactly the same magnitude as that of the wheel, but in the opposite direction. No, the earth has much higher inertia than the wheel, it will have a very small change in angular velocity. Do the calcs, you will be amazed by how small it actual is. Edited March 24, 2015 by CasualKilla Link to comment Share on other sites More sharing options...
imatfaal Posted March 24, 2015 Share Posted March 24, 2015 Bonus marks for anybody who can give a estimate of the change of angular momentum of the earth while the wheel is spinning. Make some realistic estimates for the wheel mass and angular velocity! Exactly the same magnitude as that of the wheel, but in the opposite direction. ... No, the earth has much higher inertia than the wheel, it will have a very small change in angular velocity. Do the calcs, you will be amazed by how small it actual is. Question was change in angular momentum - not angular velocity. Whole point is that angular momentum is conserved. John's answer was correct - if not very helpful Link to comment Share on other sites More sharing options...
CasualKilla Posted March 24, 2015 Author Share Posted March 24, 2015 (edited) Question was change in angular momentum - not angular velocity. Whole point is that angular momentum is conserved. John's answer was correct - if not very helpful Well they the same magnitude angular momentum with reference to the same point. You need to use parallel axis theorem on the wheel or the earth, or both if you don't choose one of their centers as the reference (not recommended). But ok you are right, I didn't realize I said angular momentum instead of velocity, my mistake. Edited March 24, 2015 by CasualKilla Link to comment Share on other sites More sharing options...
imatfaal Posted March 24, 2015 Share Posted March 24, 2015 Well they the same magnitude angular momentum with reference to the same point. You need to use parallel axis theorem on the wheel or the earth, or both if you don't choose one of their centers as the reference (not recommended). Exactly - Angular Momentum is always basis some axis; you choose the axis that makes your sums easier (and relevant). If you are studying this it would be worth while reading up about how the conservation of Ang Momentum is tied up with the isotropy of space - ie the invariance of physical laws with change of direction and the amazing theorem of Emmy Noether Link to comment Share on other sites More sharing options...
swansont Posted March 24, 2015 Share Posted March 24, 2015 If you want a fun calculation you can see if you got enough people to drive in the same direction at the same time, if you would slow/speed the earth enough that we might be able to measure it, during that interval. e.g. get several million people on the east coast of the US to drive west for 3 days at an average speed of100km/hr. What happens to the rotation of the earth? Link to comment Share on other sites More sharing options...
imatfaal Posted March 24, 2015 Share Posted March 24, 2015 Yeah - Fermi problems where you work on orders of magnitude and estimations are a superb way of checking you have got a handle on the relationships between your inputs and constants Link to comment Share on other sites More sharing options...
J.C.MacSwell Posted March 24, 2015 Share Posted March 24, 2015 (edited) Exactly - Angular Momentum is always basis some axis; you choose the axis that makes your sums easier (and relevant). If you are studying this it would be worth while reading up about how the conservation of Ang Momentum is tied up with the isotropy of space - ie the invariance of physical laws with change of direction and the amazing theorem of Emmy Noether That's why you don't even need something to counter spin...it can simply translate away, on a trajectory offset from the axis ...and then you have cat's that can land on their feet from any position, with no change in angular momentum at any point in their fall Edited March 24, 2015 by J.C.MacSwell Link to comment Share on other sites More sharing options...
imatfaal Posted March 24, 2015 Share Posted March 24, 2015 That's why you don't even need something to counter spin...it can simply translate away, on a trajectory offset from the axis ...and then you have cat's that can land on their feet from any position, with no change in angular momentum at any point in their fall Cats don't change angular momentum as a system due to the fact that they bend and twist their bodies at the same time. It is the cats flexiblity (and the wheels inflexilbilty) that are crucial - not where you measure from; a choice of measurement can never affect the result - merely make it easier to calculate that result. The most simplistic model of the cat is rotation is ironically modelled with counter spin - envisage the cat as a front and back half hinged in the middle . The cat falls feet upwards, the cat hinges in the middle (you can now see both face and tail) , the cat then twists the back half counterclockwise and the front half clockwise (with respect to you), the cat then unhinges. There is no net change in angular momentum - yet the cat is now the right way up. In reality the movement is far more complex and sinuous - but it isn't a mathematical trick, it's a physical one 1 Link to comment Share on other sites More sharing options...
J.C.MacSwell Posted March 24, 2015 Share Posted March 24, 2015 Cats don't change angular momentum as a system due to the fact that they bend and twist their bodies at the same time. It is the cats flexiblity (and the wheels inflexilbilty) that are crucial - not where you measure from; a choice of measurement can never affect the result - merely make it easier to calculate that result. The most simplistic model of the cat is rotation is ironically modelled with counter spin - envisage the cat as a front and back half hinged in the middle . The cat falls feet upwards, the cat hinges in the middle (you can now see both face and tail) , the cat then twists the back half counterclockwise and the front half clockwise (with respect to you), the cat then unhinges. There is no net change in angular momentum - yet the cat is now the right way up. In reality the movement is far more complex and sinuous - but it isn't a mathematical trick, it's a physical one Cats don't change angular momentum as a system due to the fact that they bend and twist their bodies at the same time. It is the cats flexiblity (and the wheels inflexilbilty) that are crucial - not where you measure from; a choice of measurement can never affect the result - merely make it easier to calculate that result. The most simplistic model of the cat is rotation is ironically modelled with counter spin - envisage the cat as a front and back half hinged in the middle . The cat falls feet upwards, the cat hinges in the middle (you can now see both face and tail) , the cat then twists the back half counterclockwise and the front half clockwise (with respect to you), the cat then unhinges. There is no net change in angular momentum - yet the cat is now the right way up. In reality the movement is far more complex and sinuous - but it isn't a mathematical trick, it's a physical one Right. They can't change their angular momentum without outside help (forces) Link to comment Share on other sites More sharing options...
CasualKilla Posted March 25, 2015 Author Share Posted March 25, 2015 (edited) It is very easy to rotate with no eternal forces, specially for beings with tails (such as cats or super sayans) simply rotate your tail and there you go, then stop rotating it when u have reached your desired angle. Not complex at all, cats try all kinds of things in space, but I don't think they realize that only their tails are making a difference, silly cats. Edited March 25, 2015 by CasualKilla Link to comment Share on other sites More sharing options...
swansont Posted March 25, 2015 Share Posted March 25, 2015 It is very easy to rotate with no eternal forces, specially for beings with tails (such as cats or super sayans) simply rotate your tail and there you go, then stop rotating it when u have reached your desired angle. Not complex at all, cats try all kinds of things in space, but I don't think they realize that only their tails are making a difference, silly cats. But it doesn't depend on the tail. It's about changing moment of inertia from extending/retracting legs, and the ability to have different rotational axes for front and back (FF to the 2:30 mark, if you want to skip several drops before the explanation) Link to comment Share on other sites More sharing options...
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