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Posted:You know how the poi speeds up as the cable shortens when you are doing a wrap? Well, this is an example of a principle in physics called the conservation of angular momentum."The angular momentum is the mass of the object, times the angular velocity (that is, the rate of spin), times the length of the string squared. The angular velocity remains constant. Since the mass of the object is constant, the rate of spin must increase as the string gets shorter to keep the product unchanging. So, the object spins faster and faster."--Bob Brown, Science CircusThis is also the reason why a whip cracks.Maximus
Posted:Hang on I'm now confused. I thought the actual linear speed of the wick increased as the string becomes longer as it has to cover more ground than a wick on the inside and the angular speed of the wick increased as the strings become shorter. ie within a given period of time, the long wick travels further in distance but the short wick covers more degrees. If you say that AM = m * av * l*l then if you have a 2 cm string, and say mass = 1 and av = 1 then the am (speed through the air) = 4. With a 3 cm string doesn't this become 9.Someone plse provide the definitive answer. I couldn't see it in the maths bit under force and split rings nor in a search under lenght and speed.
Posted:Maximus, I'm confused. In your post you claim that momentum is conserved (which I believe to be true) but in "Bob"'s explanation he says that: "The angular velocity remains constant." which I don't think is true.If the momentum is conserved, angular velocity remains constant, and mass remains the same, the string CAN'T change length!If you replace Bob's "velocity" with "momentum" I think it's all good. Unless he was talking about something other than this example.String length (squared) and angular velocity are inversely related BECAUSE momentum is conserved. Then, of course, you've got an elastic collison at the point of wrap which reverses the velocity but still conserves the momentum.This is also why ice skaters bring their arms inward when they spin, to speed up, and extend them when they stop, to slow down.
Well, shall we go? Yes, let's go. [They do not move.]
Posted:NYC, I think you are right. I rechecked Bob's language, and that's what he said, but it makes more sense the way you expained it. The math is certainly over my head, that's why I quoted the whole thing.Might be a way to sneak your poi into the classroom.Maximus
Posted:Ummm.. why the hell do u care what are the physics of a wrap lol.... just learn to wrap and get better at it... TALK IS CHEAP GO PRACTICE.... all of u need to do that on this site... especially the ones with like 1000+ posts lol... you sick people all u do is talk about basic moves and say what moves u just learned... go poi for 5 hours and keep it to yourself lol
Posted:Relax my fellow Gothamite. Some of us enjoy cognative discussions of the natural science as well as interacting with other human beings.I find it ironic that you posted about the irrelivance of posting. I post because I find it useful, interesting, and social. Why do you post?
Well, shall we go? Yes, let's go. [They do not move.]
Posted:I got an A in physics 1 and 2....so....The poi continues to do the same speed (as in "miles per hour") but the "revolutions per second" rpm's go up as the string gets shorter. RPM's are the same as angular velocity (the number of degrees the poi travel though in a particular space of time. You can visualize it like the poi are traveling the path of a circle, now....what is the circumference of that circle? 2 x 3.14 x length of string......so the velocity of the poi is....(2 x 3.14 x radius)/ time = velocity ("meters per second" or what ever units you want to use.) The angular velocity is the number of degrees of the arc the the poi travels divided by the time it takes to travel those degrees of an arc (radians are used instead of degrees in most physics equations) and this is the same type of measurement as RPM's (revolutions per second) which, if the poi is traveling at 10 mps on a 4 meter circumference circle, it can travel a 2 meter circumference cirlce in half that time (it will thus be doing 2 x the RPMs of the 10 m circle, but the poi will be travleing at the same speed, 10mps) So, a poi traveling 8mps on a 4 meter circumference will do 2 rpm's, a poi travling 8mps on a 2 meter circumference cirle will be doing 4 rpm's. They will both be traveling at the same velocity, and thus have the same momentum (angular momentum) but the rpm's will go up with a shorter string. The momentum of an object is the velocity times the mass. don't get confused by the angular crap, it just means everything is moving in a circle. Angular momentum is the same as plain momentum, except it is moving in a cirle. When in he quote they say "the angular velocity remains constant" that is only when the string is kept at the same lengthmomentum = A.V. x mass x L x L if the length goes down, something else must go up in order to keep the momentum the same. in this case the mass is constant, so the only thing that can go up is the A.V. (angular velocity). Angular momentum is conserved just like regular momentum, since momentum=mass x velocitythe velocity must stay the same (kilometers per second) as the poi travels in the path of a circle (just like a car driving in a circle and you look at the speedometer and it says 10 mph). if the path of the car makes a smaller circle it will still be going 10 mph, but it will complete one circle in less time. Keep in mind that angular velocity and velocity are different things. angular velocity in physics simply describes the number of radians the object passes through (360 degrees = 2 radians) in a certain amount of time, which is similar to revolutions per second. Regular velocity is the distance (not rotations) that an object travels in a certain amount of time.Daniel Tyler
Posted:Did someone say physics? You ought not to have done that, because there may be a physicist or two lurking...Oh look, here's one now! (writing my PhD dissertation presently)conservation of momentum is basically one of the field of physical mechanics statements of conservation of energy. Energy is never created or destroyed, it only changes form. That is one of the few rules of physics that you can't disprove on some level.So then, if angular momentum of the poi is conserved whilst spinning, which it is if you ignore friction in the chain links, air resistance, torque from the spinners body, and what have you, then we can say angular momntum is a constant and doesn't change. so, if:AM = angular momentumRI = rotational intertiaRI = constant*mass*radius^2 (that's radius squared)av = angular velocity then AM = RI*av so if AM and mass are constant, and we make all constants = 1 for simplicity of view, then:1 = radius^2*av so then, as radius gets smaller (you wrap your poi), then av must get larger if the above equation is to hold true.as for linear velocitylv = radius*avwe can see from the avove that the av of a poi spiraling in must obey:1/radius^2 = av and also:av = lv/radius thus,1/radius^2 = lv/radiusand solv = 1/radius for this situation.so then, linear velocity actually gets smaller with increasing radius, and the angular velocity get smaller faster than the linear velocity with increasing radius.Again, this is specifically for this physical setup (all you are actively changing is the radius - all other changes are physical consequenses of that action). If, for example, you use the case of an object sprialling into a star because of gravity, then we have to make a completly different argument and the results will be different. And I have dropped constants here, so this is actually a proportionality argument instead of strict equations. There are constants missing in the above arguments, but the general trends hold true.QEDI think I'll use some poi next time I teach angular momentum to a class. -v-[This message has been edited by vanize (edited 03 February 2002).]