M : moment
From Introduction to violin physics
This is show the how the momentum will occur while people play the violin. When you put down the right hand on the bow, each fingers will give a force to down. However, the thumb, which put between the band, does give force up. This will happen at the frog of the bow.
x :
distance from fingers to string
P :
pressure
a :
distance from fingers to the center of gravity of bow (const.)
W :
the weight of the bow (const.)
From Introduction to violin physics
The bow is moved a long at a particular speed by the right hand. Frictional forces between the bow and the string alternate between sticking and sliding. And the pressure will occur at the mid-end of bow where it meets the string. As the picture shown, it will be the free-body diagram for bow.