# How fast can a rigid diak spin without disintegrated?

1. Mar 4, 2004

### Sammywu

Did any one think of or have answered this question?

When we spin a rigid metal disk, shouldn't the atom at the edge of the disk tends to fly away in a linear motion?

The solid atomic structure must act as a centripetle force to keep the atom in place.

If we keep spining the disk, will the disk eventually disintegrate, because the atomic force is no longer greate enough to keep it in one piece?

2. Mar 4, 2004

### Njorl

There is more to it than that. True, the atom at the edge experiences the greatest acceleration, but the only mass pulling at the bond is the mass of that atom. The atom just inside of that one has it's own mass, and the mass of the atom to the outside experiencing essentially that same acceleration. So, the bond holding this atom to the disk experiences roughly twice the force. This is even further complicated by the 3 dimensional nature of the structure.

You could probably work out a minimization problem based upon a total energy comprised of rotational kinetic energy, and potential energy due to compressive strain along the axis of rotation and expansion in the plane of rotation. The compressions and expansions at each point in the disk could be solved for as a function of rotational velocity. If the strains at any point indicate a breaking of the lattice, the disk will disintegrate at that point. Whether that point is at the edge, the center or in between is not obvious to me. My guess is somewhere in between.

Njorl

3. Mar 5, 2004

### pallidin

Agreed.
I would also like to offer a word of caution.
Rotational forces and the potential for fragmentation is nothing to mess with, without a full understanding of what is going on.
Take a simple Dremel tool for example, which has rotational speeds of 20-30,000 rpm's and above(depending on model) A 1-inch disk especially designed to work with the Dremel tool requires care but not paranoid caution. However, attempting to rotate a 6-inch pencil or other non-approved attachement at high speeds can cause serious injury, possibly death, due to fragmentation from bond-breaking, as the stresses created on a larger diameter "attachment" are very, very high.

4. Mar 14, 2004

### Averagesupernova

Cool, let's try that pencil thingy you were talking about. Or we could suggest they do it on that show jacka.

5. Mar 15, 2004

### Staff: Mentor

FYI, computer cd-roms are limited in speed by this very issue. Some of the faster ones have had issues with cd's shattering due to the internal forces on the cd.

6. Mar 15, 2004

### outandbeyond2004

If you spin anything fast enough, it will disintegrate. Whether there is some kind of material that cannot be spun fast enough by any practical means, I do not know.

7. Mar 15, 2004

### Michael D. Sewell

In the 1950s many drag racers suffered foot and leg injuries caused by clutches and flywheels disintegrating at high rpms. In the 1970s at a tractor pull, a man in the audience was killed by a piece of a water pump, in Denver as I recall.
Sometimes a water drip is used for cooling on a grinding wheel. It is important not to run the drip until after the wheel is turned on. Some wheels can absorb a lot of water, and will come apart.
Please be careful around anything that spins at high rpms, especially if it is made of a material that is brittle or there is any chance of it being out of balance.