Wednesday, February 25, 2009

Mad science: Slasher physics.

The Guardian has an article on the work of one Dr. Sarah Hainsworth, of the Space research Centre of the University of Leicester: the Fredrick Winslow Taylor of the slasher world. The good doctor is studying just how sharp (and sharpish) things poke through the human body. From the article:

Dr Sarah Hainsworth is an engineer who, for the past few years, has been investigating the sharpness of knives - and not just knives, but screwdrivers, scissors and even ballpoint pens. Any implement, in fact, that has featured as a murder weapon. "If you give somebody a knife and ask them if it's sharp, how do you measure that? How do you quantify sharpness?" Hainsworth asks. "I suppose that's what we've been trying to do."

The cutlery industry has measured the effectiveness of the slicing edges of blades designed for chopping up vegetables or carving meat, and how best to maintain that sharpness, she says. "But what nobody has really done is to investigate the sharpness of points." They certainly have now. Hainsworth sometimes spends whole afternoons dropping knives from various heights on to foam or legs of pork - a close substitute for human skin - and recording the results in finest detail.

Hainsworth found herself in this curious specialization after she was approached by a solicitor who thought that data on the amount of force his client used in a stabbing might influence sentencing; a perp who used less force, the thinking went, was not trying to hurt somebody as badly as somebody who went all crazy with the stabby stab. For those of the slasher-fan persuasion, however, the results are essentially a close-up, slow-mo take on the work on your favorite villains.

Using high-speed video film, they have gained a better understanding of the "mechanism" by which blades penetrate skin. This shows that there are a number of stages. At first contact the skin deflects around the knife tip. At a critical stress level the knife penetrates the skin. After penetration the sharpness of both tip and blade are important in determining how much further the knife goes in.

Stress is an engineering term expressed as force, measured in Newtons divided by area, Hainsworth explains. The force required to stab a person with a sharp knife is less than you might expect, she says. "Often, once the point has gone through the skin, the force required to go further is even less."

. . .

Other factors come into play in a stabbing, for instance the angle of strike and the width of blade. In further research, Hainsworth will replicate knife attacks using an accelerometer, a device that records the rapid changes in force used.

In her experience, the kitchen knife is the weapon most commonly used in murders. In recent years, more and more of these have been made with sharper tips - not necessary for the slicing and peeling most are designed for. Hainsworth suspects this is because people buying knives are often seen to check out the tip on their thumb rather then the long edge, and manufacturers have noticed this tendency.

1 comment:

Anonymous said...

For paring knives, maybe you want a sharp point (eg to core an apple).. Though you might end up like the Mrs and get a few stitches in the palm while doing so.

For anything else, you need a sharp blade, and who cares about the point of the knife..

And there's something to be said for simple blunt force of not-to-sharp objects.. Machetes operate on the force principle, not sharpness..

(oh yeah, can view from werk now. Perhaps whatever set off the word-filter here rolled off the front page)