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Re: why do scalpels cut easily when moving?



The edge of the scalpel is a wedge. When you press straight down you have
the mechanical advantage of the full wedge angle. In effect you look
at the angle made by the surface in a plane perpendicular to the edge.
When drawn sideways the effective angle is the angle made by the surface
in a plane at a much reduced angle to the edge. This smaller angle gives
a much greater mechanical advantage. (The mechanical advantage is
proportional to the inverse of the tangent.)

I certainly don't know a good answer to this question, but I
interpreted it differently. When I gently press a scalpel straight
down on flesh without sliding it will not cut. Yet if I slide the
pressed scalpel in the direction of its edge along the flesh,
without changing the angle made by the surface in the plane
perpendicular to the edge, it will cut easily, and will do so even
with a lesser downward force. The scalpel blade is exceedingly
sharp (for some purposes surgeons even use freshly knapped
obsidian blades) and the difference between a sharp scalpel and a
knife is considerable. I too would like to know the answer to this
question.