The Mechanics of the Bow, or why long slow bows do so much good.
We can think of the bow as having two fulcrums: the thumb and the string. The thumb is a fixed fulcrum, while the string is a moveable fulcrum.
The weight of the bow is shared between string and thumb. As the bow moves on the string, the ratio of weight on string to weight on thumb changes.
The bow is weighted assymetrically - the centre of mass, or balance point is about a third of the way up from the frog.
At the tip, there is about twice as much weight on the thumb as there is on the string.
About a third of the way from the tip, the weight is shared evenly between thumb and string.
At the balance point, all the weight of the bow is on the string.
Past this point, closer to the frog, the bow would tip over the string if we didn't put more weight into the bow at the frog end to balance it.
However, such extra weight crushes the string and prevents its vibration, so we have to take the weight off the string.
There are two ways to take that weight off. One is to increase the weight from the little finger of the right hand, thus raising the bow about the fulcrum of the thumb.
The other is to lift the whole right hand, effectively taking the weight of the bow into the hand.
Long slow bows teach us to feel these transitions of weight from string to right hand.
Of course, there are two other main factors involved in the mechanics of the bow: the contact point on the string, or distance from the bridge, and the speed at which the bow is moving. Both of these parameters have an effect on how much weight the string can take before the bow stops the string's free vibration. As a rule, the string can take more weight closer to the bridge, and it can also take more weight if the bow is moving more quickly.
When moving more quickly, we have to add weight from the arm to keep the bow hair connected to the string.
Pronation of the right hand, or leaning into the stick of the bow with the first finger, is one method of increasing the weight being transferred from the arm into the bow. Another way is just to relax the right arm, and its weight comes though the whole hand, directed by the index finger.
In my one-to-one lessons on bowing, I show you exactly how to build your tone and bow control, by gaining mastery of the bow's weight, speed, and point of contact.