Why is chalk left on the blackboard?

Why is chalk squeaking on the blackboard?

A not so popular sound is the loud squeaking of chalk on the school blackboard. How does the squeak come about?

Chalk can squeak quite loudly on the blackboard

Many students and adults find the squeaking that sometimes occurs on the school blackboard when drawing and writing with chalk as extremely unpleasant. Some even cover their ears, others even get goose bumps. When we think about why it squeaks so loud, the question is not that easy to answer.

In our ideal, the following happens: If we draw a line on the blackboard with chalk, the chalk glides evenly over the blackboard. The slightly rough surface rubs off some material at the tip of the school chalk, which remains as a visible drawing on the blackboard. At some point the chalk is used up and you take a new piece.

In reality, however, the following can happen: The frictional force holds the piece of chalk firmly in place while you are drawing. Depending on how hard the chalk is pressed on and at what angle you hold the chalk, the chalk sticks to the board for an extremely short moment. The power of the person drawing moves the chalk relentlessly on, the chalk slides a tiny bit further on the board, sticks again for a moment, etc. This interplay can be continued as long as you like, as long as the same conditions prevail. Since the process takes place very quickly, the chalk creates vibrations in the air, which we perceive as sound, as the squeak. The blackboard intensifies the squeaking because it acts like a sound box

If the conditions change (you increase or decrease the force or the speed, then this mechanism ends immediately. Likewise, the squeaking stops immediately when the chalk comes into a damp place (after wiping the board). This mechanism is because of the interplay of sticking and sliding is also known as the "stick-slip mechanism".

The same effect is also responsible for the squeaking of car tires, the sound of a violin string, the squeaking of a tram in a curve or the creaking of a door. The popular squeaking of a fingernail that rubs across the board is also based on this principle.
 


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