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Establishing the Maximum Speed of a CD-ROM Drive
UnknownSoldier writes "Ever wondered how fast CD-ROM drives can spin their CDs before the CD will self destruct due to centrifugal force? This person was too, and has his results. (So much for those 100x drives)."
- 6 replies here and the site already is slashdotted.
Anyway, I think you can make cd drives that spin 4000x if you want, because it might be possible to put the cd in braces to hold it together, and/or to rotate the laser instead. Or how about using multiple lasers?
It's just like silicon transistors: There's always somebody saying there is a final physical limit we'll reach within the five years...
Often, we(they)'ll find a way around the limitation.
--- Hindsight is 20/20, but walking backwards is not the answer.
The slow seek time doesn't bother me nearly as much as the eternity it takes from the time you insert the CD in the drive until the time it is ready to send data. In fact, I'd probably be happy with an 8X drive if it had a < 1 second delay between hitting the close button and viewing the README file.
The beam will bounce off the pit and either scatter or reflect back up into the mirror striking the focal point
:o)
Thats how you learn how CD`s work in school, but it isn't true. In past it was the classical approach of not telling the whole truth to keep others from copying it.
First the beam is not scattered or reflected, it is _always_ reflected. The CD consits of two layers, the back one is solid and 100% reflective. The distance between the two layers has to be exactly lambda / 4 of the lasers wave length. Now the first layer is semitransparent. Meaning 50% of the light gets through 50% gets reflected. In the first layer you have the pits representing the data. If this layer has a pit 100% of the light gets reflected, but if it hasn't only 50% get through, get reflected at the back layer and then has a destructive interference with the light reflect first. (That's why the distance has to be wavelength/4)
I fear that the interference will not work if the light is not angeled with 90 degree on the disk.
How about using 700 Million lasers, not spinning at all? You could read a CD at once
--
Karma 50, and all I got was this lousy T-Shirt.
Sorry to be a physics geek here, but there's no such thing as "centrifugal" force, unless you're talking about the force caused by a centrifuge dropped from a height.
There IS "centripetal" force, that refers to the force on an object travelling in a circle, which pushes outward from the axis of said circle on an object while it's travelling about the radius.
centripetal force is a force acting toward the centre. in the stone on a string example, it is the force (tension in the string) pulling the stone toward the holder of the string, making it move in a circle. nothing is "travelling about the radius", and nothing is pushing outward from the axis. strings don't push!
centrifugal force is something you get in rotating frames of reference. one doesn't normally use such frames in physics because they are unecessarily complicated. but that is just a matter of calculational convenience; centrifugal forces are real enough in a rotating frame (it is called a fictitious force because it depends on the choice of frame, rather than being intrinsic. see this page). take a fast curve in a car and that fictitious force feels real enough, even if it isn't the simplest way to describe the situation mathematically.
Everything should be made as simple as possible, but not simpler. -- A.E.
There is no "centripetal force." There is, however, a centripetal acceleration, which points *inward*. (Look up the word) You're committing the classic mistake of confusing a force with an acceleration. For example, in your example, the ball's centripetal acceleration is inward. By Newton's 2nd law, a force must be acting on it. The force in this case happens to be the tension of the rope.
There IS centrifugal force. It's a fictional force, which is a sort of misnomer. A fictional force is nothing but a force felt by an object in an accelerating frame of reference, like a ball on a string (since velocity is changing direction), or a car getting on a freeway (since velocity is increasing). The fictional force in your example would be the one felt by the ball, radially outward, with magnitude equal to the tension on the rope.
I think it is you who should have paid attention in physics 101.