The action of the bow which drives the strings is a regular cycle of stick-slip-stick-slip. This involves some interesting properties of friction, the force that makes things difficult to slide. If you have ever tried to slide a heavy object such as a piece of furniture, you will know that it is easier to keep it moving than it is to get it moving in the first place. In this case static friction (sticking) is greater than kinetic friction (sliding). This is true for most dry surfaces. It is also true of the string and the bow, and the player puts rosin on the bow to give a big difference between the two conditions: the coefficient of static friction is high, while that of sliding friction is very low. (Note that, in the animation below, the vertical scale of the string's motion has been greatly magnified, and the wave speed in the string has been reduced by the same factor. There are also many complicating factors, including one due to torsional waves.)