Chalk squeal and—shudder—fingernail screech occur when frictional forces aren’t constant as chalk meets board. Friction is the force that puts on the brakes on an object in motion. Scientists define friction as “the force between two solid surfaces that resists sliding.” The main cause of friction is an overly friendly relationship between the atoms of two surfaces (like the bottom of a heavy bookcase and the floor). At the spots where two such objects make contact—called junctions—their atoms have formed bonds.
When we drag chalk across a blackboard at certain angles and with enough pressure, it sticks to the board, slips, jumps to a new spot, and then sticks again. Scientists call it the stick-slip phenomenon. The chalk may stick and slip hundreds of times each second. Since each jump covers such a tiny distance, we don’t see gaps in the letters written on the board.
But each time the chalk sticks, it emits a burst of noise, which gets louder if the chalk is pressed harder. Researchers say the screech occurs because—where chalk meets board—the constantly varying frictional forces excite the natural resonances of the chalk. The chalk’s vibrations are reinforced (they resonate), becoming stronger. So as the chalk stutters across the blackboard, it makes a squealing sound. The slipping and sticking also occurs when you drag your fingernails down a blackboard, which usually tops the list of sounds that make us flinch.