Why Do Violin Bows Use Horsehair?

Violin bows use horsehair not because the hair itself “sings”, but because it can form a light, flexible ribbon that can be tensioned and replaced. A modern violin bow usually carries roughly 160 to 180 hairs side by side. Rosin applied before playing gives the contact between hair and string enough controllable friction to work.

As the bow moves, the string briefly sticks to the hair and travels sideways with it, then slips rapidly back under its own tension. This extremely fast stick–slip cycle keeps feeding energy into the string, so a note can be sustained rather than simply decaying as a plucked note does. The important thing is the contact system formed by horsehair, rosin, bow tension and the player’s movement—not a vague idea that the hair alone is “rough”.

Listen to how one sustained note changes as bow speed, force and contact point shift: it can move from soft to bright, or break into noise. Oily or worn hair, or uneven rosin, narrows that controllable range. Synthetic hair can also work, so horsehair is not a mystical material. Its enduring value lies in combining flexibility, durability, rosin-holding capacity and replaceability in one very light ribbon.

https://www.yamaha.com/en/musical_instrument_guide/violin/mechanism/mechanism003.html
https://www.yamaha.com/en/musical_instrument_guide/violin/mechanism/mechanism006.html
https://phys.unsw.edu.au/jw/Bows.html