Why Does Chalk Squeak on a Blackboard?

When chalk moves smoothly across a blackboard, friction usually produces only a faint scraping sound. The piercing squeak appears when the tip enters a rapidly repeating stick–slip cycle. Static friction briefly holds the tip while the hand keeps moving, so the exposed length of chalk bends slightly and stores elastic energy. Once the force exceeds what the contact can sustain, the tip slips, the chalk springs back, and contact sticks again.

This cycle can repeat many times each second, creating a more regular vibration. Both the chalk and the board respond. When the cycle readily excites one of their vibration modes, the motion is amplified into a squeal. The sound is therefore not simply a case of “more friction means more noise”; it is a self-excited vibration produced by frictional behaviour, elasticity and resonance working together.

A longer length of chalk outside the fingers bends more easily and can store more elastic energy. Pressure, writing speed and the angle to the board also alter the stick–slip cycle. Holding the chalk closer to its tip and moving it steadily at a lower angle often weakens the squeak because the shaft no longer behaves as such a flexible little beam. The method changes the vibration conditions rather than eliminating friction.

Related mechanisms occur when windscreen wipers judder or brakes squeal. The materials differ, but the contact can repeatedly grip and release, converting a steady push into periodic vibration.