How fast can you spin a rod around? The speed of light?

The limit is closer to the speed of sound, not light. Let us explain.

An object moves in a straight line at a constant speed unless some force acts on it. In this case, the end of the rod you're swinging actually tries to go in a straight line, but you're exerting a force to pull it into a circular path.

Even if you're very strong, there's a limit to how quickly the information from your arm can get to the far end without the rod breaking, and that limit is roughly the speed of sound in the rod. In steel, the speed of sound is about 5,000 meters per second - much, much slower than the speed of light, which is 186,000 miles per second.

That gives you a rough idea of how quickly the end would have to be moving before the rod would break. If there are weaknesses in the rod, it would break at much lower speeds. This is an important concern for flywheels, propellers, and other objects that spin around rapidly.

Dr. Knowledge is written by physicists Stephen Reucroft and John Swain, both of Northeastern University. E-mail questions to drknowledge@globe.com or write Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819.