HOW COME?: Chalkboard's squeak

How come when you write on a chalkboard it sometimes makes that screeching noise? asks Brendon Allen, a student at Long Island Lutheran Middle and High School in Brookville

Fingernails scratching down a blackboard -- it's considered one of the most irritating sounds in the world, according to surveys. But if survey takers were asked about squeaky chalk, they'd probably rank that sound in the top 10, too. In fact, the noxious noises share a common cause.

Here's a clue to what makes chalk squeak: As it makes that cringe-inducing screech, it leaves a series of very tiny dashes on the chalkboard rather than a smooth, unbroken line. Scientists say the explanation involves both friction and resonance. Chalk screams fill the air when frictional forces at the point where chalk meets blackboard aren't constant.

The definition for friction is "the force between two solid surfaces that resists sliding." In other words, friction is the force that puts on the brakes. The main cause of friction is an overly friendly relationship between the atoms of two surfaces, such as the bottom of a bookcase and a wood floor. At the spots where the floor surface and the bottom of the bookcase make contact, called "junctions," their atoms have actually formed bonds.

How does it work with chalk? When we drag a piece of chalk across a blackboard, it may write smoothly, making a not-so-unpleasant sound. But hold the chalk at the wrong angle and with pressure, and instead of gliding it sticks to the board.

However, each "stick" is quickly followed by a "slip." And as the chalk sticks, slips to a new spot and sticks again, it leaves a series of tiny marks rather than a continuous line. (Since each jump covers a small distance, we don't see gaps in the letters or numbers written on the board.)

Scientists call this -- surprise -- the "stick/slip phenomenon." And each time the chalk catches on the board, it emits a burst of noise. The harder the chalk is pressed to the board, the louder (eek) the sound.

But why, exactly, the screech? Scientists say chalk squeals because its own natural vibrations are heightened by the stick-slip forces at the point of contact. Think about pumping your legs on a swing. If you pump at just the right frequency, the swing's back-and-forth movement is reinforced (or "resonates"), getting you higher off the ground with each swing.

Likewise, when chalk stutters across a blackboard, its vibrations are reinforced. Presto: Chalk squeals.

The world is full of examples of the slip/stick phenomenon. A rusty metal drawer being pulled open. A butter knife drawn across a china plate. The screech of subway train brakes. Or two balloons rubbing together.