Bow and String Interactions

The primary reason why bowing works to drive the vibration of the strings is because of the difference in static and kinetic friction. When the string is moving in the same direction as the bow, static friction helps to accelerate it, or maintain it's speed. When the string is moving in the opposite direction, the kinetic friction slows the vibration down, however, because it generates a smaller force, the string is still allowed to accelerate with constant motion of the bow. This is why players apply Rosin to the strings. Rosin has a high static coefficient of friction and a low kinetic one.

http://newt.phys.unsw.edu.au/jw/Bows.html

String Vibrations

Once the strings are vibrating, they are producing sound. The amplitude of their vibrations (the amount that the string moves with the stick phase of the bow) determines the relative volume. Therefore, applying more downward force with the bow (normal force), will cause the string to travel farther with the bow and have a larger amplitude. Pushing a finger down on the far end of the string moves the endpoint node farther in, decreasing the period of vibration, which causes the pitch of the note to be higher.