Re: Number of RPM's of any object possible or recorded.

Hallo Donna, and thanks for the question!

How fast can anything rotate, you ask! 

Formally, the greatest rotational speed of an object (let's say it's a 
disk, although it makes no difference if it is an irregularly shaped 
object) is limited by the constraint that its fastest particles (those 
beeing the ones along the rim) must move at the speed less than that of 
light (designated by c and ammounting 3e+10 cm/s). Hence, if a disk has a 
radius R, its limiting angular speed OMEGA is given by 

              OMEGA < c/R                                    (1)


Thus, and again formaly, reducing the size of the object you can achieve 
any angular velocity. 

However, there are two problems with this reasoning. 

The first problem is that whan you go to sufficiently small objects, you 
enter the quantum world where the whole notion of a rigid disc, as well as 
its rotation becomes quite blurred. So basicaly, the first problem is that 
there is a limit to the size of the disc which still makes discussing its 
rotation, in a common sense of the word, sensible. Let's suppose the 
smallest sensible rigid disk we can build with todays nanotechnology is 
about ten to hundred times the size of the atom, which is about 10e-5 to 
10e-6 centimeters (I think that with this judgement I have greately 
exagerrated, but let's take it for the sake of the argument). Left aside 
the problem of how would you actually spin such a disk, the maximum 
angular velocity of such a disk according to the equation (1) would be of 
order of 10e+16 revolutions per second.

The second, and much more relevant problem with rotation of any material 
object is the centrifugal force which tends to dismember the rotating 
object. You probably know what it feels like going 'round on the marry-go-
round. Now imagine that the m-g-r of 10 meters in diameter starts spinning 
once a second. I bet that, standing at its rim and holding on to a steel 
bar attached to it, you would not hold on to it for much more than couple 
of tenths of a second or even less. I am not suggesting you would be 
affraid of the fast rotation, but your hands could simply not provide 
enough force to enable you to hold on. The same goes for any rotating 
body. Since there is an upper limit to the force between any two particles 
in the rigid body the particles along the rim simply cannot hold on to the 
rest of the object if it's spinnig too fast. 

Now to thew fastest spin ever observed for material objects. You might 
have heard about the extremely dense astronomical objects called neutron 
stars. If not, never mind, I'll briefly explain what it is. A neutron star 
is a remnant of a burned out massive star, which has exploded, expelling 
its outer layers into space and the remaining core collapsed to an object 
composed exclusively of neutrons whose density is 10e+14 grams per cubic 
centimetre. These neutron stars have a habit to rotate VERY fast, due to 
the fact that they inherit almost the whole angular momentum of the ex-
star. Since stars have radii of some 1e+7 km and spin roughly few times a 
month, while neutron stars are typically some 10 km side-to-side, the law 
of conservation of angular momentum should give you an idea of HOW fast 
this rotation is. The fastest pulsars noted up to date are called 
milisecond pulsars, and rotate (as their name suggests) some 1000 times 
per second. The force preventing such an object from dispersing due to the 
fast rotation is GRAVITY (as a homework I leave to you the calculation of 
the gravitational force on the surface of the pulsar!).

The fastest rotating objects made by us humans are (I guess, as I'm not an 
expert on the fiels) electromotors, and these achieve, compared to 
pulsars, a modest angular velocities of some 50 Hz (times per second, that 
is...).

By the way, if you want to learn about pulsars, check out the WEB
adress:

 http://pulsar.princeton.edu/

Hope I've been helpfull!

Duje Bonacci




Admin note:
We received the following additional information from Herb Multhaup:

Duje Bonacci contends that an electrical device rotating at
50 HZ is a maximum for a man made machine.  Multiply
by 60 seconds per minute and you get 3000 RPM, which
were the units in which the question was asked (what is
the max. possible RPM?).  Virtually any modern car
engine exceed that value (often without exceeding the
speed limit).  The Honda S2000 redlines at 9000 RPM.  A
small gas turbine may operate well over 10,000 RPM.
The expert is correct; the smaller the device the faster it
may go without failing, if all else is equal.