Why do scissors perform so well (compared to knives) when cutting hair/paper, with much flater blade angle? Is the angle flatter in scissors in order for them to become dull slower?
I'm under the impression that highly angled knives should be sharper than flat angled knives.
 A: Presumably the phenomena follows a description like this:




Step
Scissors
Knife




1
The operator presents the scissors to the hair, holding the hair slack so that the point to cut is stable
The operator presents the knife to the hair, holding the hair taut to produce tension to try and maximize the knife's cutting edge to shear the hair


2
The blades first contact the hair, producing a static friction force, and grabbing the hair
The blade contacts the hair, the tension between operator and scalp increases, but the amount of stress produced is minimal because while sharp, it is only operating on shear


3
The blades clamp together, physically pulling the hair apart, exceeding the ultimate tensile, or shear stress of the material
A few hairs come out, and the upset client scolds you for trying that!




Both modes use a narrow blade which raises the density of force employed. Using the knife limits the interaction only to shear strain (cutting), wheras the scissors combine both shear, and tensile (tearing) strain.
The flatter blades might contribute positively to the friction "grabbing" described, but this should at least introduce some of the terms used to discuss this matter.
