Nov 062011

Easy, just beam up from the Halkan planet during an ion storm and come aboard the “mirror” Enterprise!

Actually there is another way that really works, but the real message here is about examining things that we take for granted during our day-to-day lives. First off, what makes a pair of scissors “handed” in the first place? It’s more than just a molded handle that fits comfortably into a right hand or a left hand. It has to do with the topology of the shears so that they are held together during closing.

Consider how your right hand closes when you cut with a pair of scissors. The handles apply force across a revolute joint which acts as the fulcrum for two levers. But there is more going on than just a simple up and down movement, and this is the key to “handedness.” Your hand is applying a torque or slight twisting force across the fulcrum which helps keep the shears held together. Where does this torque come from? It’s part of the movement your hand makes as you close the scissors.

A pair of scissors is held between the thumb and the fingers when cutting. When the scissors are fully closed the thumb is on the outside of the fingers. Thus the hand is essentially forming a fist, the most fundamental motion of a hand with an opposed thumb.

Imagine a small rod held between the inside of the thumb and the outside of the fingers as you make a fist with your right hand. Which way would it rotate? With a little bit of visualization you can see it would rotate clockwise, as seen from above.

Conversely, the same rod held in the same position of a left hand would rotate anticlockwise, as seen from above.

Now we get to the secret. Notice I qualified the assertion about the rotations with “as seen from above?” That’s because if we see a rotation from the other end, or from below, it is seen to rotate in the opposite direction. Besides the shape of the handles, a right handed scissors is designed for a clockwise torque and a left handed scissors is designed for a counterclockwise torque, as seen from above.

Instead of beaming into the mirror universe of Star Trek, we can “see from below” by turning the shears around! When right handed scissors are held normally in a right hand, the applied torque is clockwise as seen from the revolute joint. When held with the blades facing backwards in the right hand the torque is applied counterclockwise as seen from the revolute joint. The best way to understand these issues is to simply hold a pair of scissors in your hand and feel for yourself the forces involved. Holding a pair of shears backwards is totally goofy for practical purposes, but the goal here is to illustrate what’s really going on.

 Posted by on 11/06/2011 Science

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