I understand that friction causes the chaos dust to come off the stick of chalk, but what exactly is causing the chalk dust to stay on the chalk board until rubbed off?
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1$\begingroup$ That's an interesting question and it took a bit of googling, but I found a possible answer here: wikihow.com/Clean-a-Chalkboard - Tiny peaks and valleys. $\endgroup$– userLTKCommented Nov 7, 2015 at 7:26
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2$\begingroup$ 'Chaos dust'... sounds brutal! $\endgroup$– nluigiCommented Nov 7, 2015 at 9:38
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2 Answers
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Friction causes the chalk to stay on the chalkboard. While the chalkboard appears smooth, under a microscope its surface is rough. Chalk is a much weaker material than the chalk board. When it is forced across the chalk board, small parts of chalk ('dust') are broken and remain trapped by friction in the surface asperities of the chalk board. The rougher the chalkboard, the harder the chalk board is to clean because chalk gets trapped 'deeper' in the chalkboard.
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3$\begingroup$ Word of the day: asperities! $\endgroup$ Commented Nov 7, 2015 at 12:44
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$\begingroup$ This answer is incomplete. What provides the normal force for friction to work? (It's mostly not gravity, since the chalk marks would stay in place even if the chalkboard is held face down.) $\endgroup$ Commented Nov 12, 2019 at 23:29
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Due to the adhesive force between particles of chalk and particles of board, particles of the chalk stick to the board.
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1$\begingroup$ Try elaborating your answer to justify your point. $\endgroup$ Commented Aug 25, 2020 at 8:45