Rotational mass versus static mass

[1BADDSM]

I'm too lazy to open my physics book up and come up with my own ratio of rotational mass versus static mass, and there are too many people who came up with their own ratios (1:1.4, 1:4, 1:5, sqaure, ...) which is correct? Can you explain how you derived it or prove it in a logical manner? If not I'll go bother my old physics profesor, he should know or be able to come up with the correct answer fast. :)

~jm

[Ghislain]

Hey!!!

Dave Coleman (Sport Compact Car Mag) had a few interestings articles about Rotational Mass.

Try to look up the old articles, you could find some cool info about that topic.

You might be able to find them on their site;
http://www.sportcompactcarweb.com

Hope this helps.

Ghislain.

[TRST165]

The mass does not change, whether rotating or static..... Are you talking about rotational Kinetic energy? The Rotational kinetic energy of a rigid object = .5 x (moment of inertia) x angular speed squared. That what you where looking for?

[4Banger]

Its harder to move a rotating object than it is to move a static one, but there is not constant rate in which that occurs, it will depend on the object's size, weight, rpm and how the weight is distributed on it. (try to imagine a bicycle wheel spinning at 50rpm's, now imagine a object of the same size, spinning at the same speed but with most of it's weight concentrated at the center of the wheel, the tire will be much harder to move even though they have the same size, weight and speed). I know this answer didn't help a lot, but at least I'm trying to give you an idea of how it works. :o

[1BADDSM]

I thought I posted something up here already after TRST165 did.... My computer is a WHORE!

Anyway, yeah that makes absolute sense that it's where the weight is concentrated, but on a real rim you can assume that it is concentrated on the outside of it for rims (should be able to use formula for hollow cylinder, right?!). I guess where it'd be worse is really the (estatic?) force to get it moving from a stop really, so when w= 0+ I have a pretty good grasp on the concepts of physics I'm just not very good with the formulas.

Thanks for the inputs so far,
~jm

[4Banger]

I guess where it'd be worse is really the (estatic?) force to get it moving from a stop really

Not exactly, there are two different forcess involved here, one is the one you just discribed (the force required to make the object spin), the other one is kinda hard to explain, it'll happen when the object is spinning (gyroscope effect), and it'll get worse as the rpm increases. I'll try to give you one exemple; Let's use two equal bicycle tires (they are not touching the ground and they are both static), one is spinning very fast, the other one is not spinning, you'll need much more force to move the one that's spinning than to move the one that's not. Whant proof? get one of those drywall cutters that spins at 30,000rpm and wigle it around, you'll be amazed. (a dremmel will give you the idea, but it's not really convincing) :beer:

[surfingpimp411]

Static mass is just that how much is in an object while it as at rest. Sit in that swivle chair at your computer and spin your self with you arms out streched, now pull you arms inward notice how you just picked up speed. That's because you just lowered your rotational mass. Your compact around your axis of rotation so you spin faster it's the principle that under drive pulleys work on. the same amount of force that is used to spin you with your arms outstreched can spin you much faster once you compact yourself around you axis of rotation simply put force x mass = acceleration. The lower the mass the more the acceleration. that's why dragsters have such small skinny wheels up front less energy is wasted on getting them moving. I could write a paper on this but i think this is oversimplified but I hope it helps a little.
4banger should have watched more Mr. Wizard as a kidan object is harder to turn when it;s weight is concentrated on the outside of it's diameter due to gyroscopic effet( I know I can't spell). prime example a gyroscope all the mass is on the outside with almost non in the middle how else could those little suckers balance on a string.

[dsm4eva]

While it has been a few years since I've used my dynamics skills, the statements above about required energy to move a mass with a larger radius of gyration versus one with a smaller radius of gyration is incorrect. A smaller radius of gyration will require less energy to rotate but will also yield a lot less momentum. The energy input required is easy to visualize as it is a component force vector at the radius of gyration, which is a torque input. Smaller torques means less energy.


And then I read surfingpimp411's post and realize he said everything already....