When writing on a black board, why does the chalk make that awful squeaking noise? And why should the angle and whether you push or pull the chalk make a difference?
I'm almost sure there has already been a thread about it sometimes ago.
The reason is chalk friction with the board: the chalk accelerates fast, then stops then accelerates again, many times every second, making it vibrate at high frequency. So, angle, push, but also chalk lenght
, affect the kind of sound (or not sound).
Found that thread. Guess who asked that question? http://www.thenakedscientists.com/forum/index.php?topic=9332.0