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The Kid Should See This

The engineering behind the scissor gate and “scissors NOT gate”

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Scissors are made from two one-sided blades that are connected with a pivot screw (hinge pin). Its ends lift up or drop down when pressure is applied to its handles.

When linked in a series, as shown above, this scissor hinge configuration can create a folding gate that expands (or closes) and contracts (or opens). This mechanism is also used in grabber toys, extendable lamps and mirrors, and large scissor lift construction vehicles.

scissor gates
In the video above, Dr. Henry Segerman demonstrates how the scissor gate works… and how one change within the links can create a “Scissors NOT gate” mechanism: when one side contracts, the other expands, and vice versa. Segerman narrates:

“So how does it do this? How come this NOT gate scissor linkage works so differently than the standard scissor linkage?”

“Well, so the first thing to notice is that the first two X’s here and the last two X’s here, they’re identical to the standard scissor linkage mechanism. The whole difference is happening in the middle here. And you can probably just see it:

“There are these L-shaped pieces instead of the usual straight across from the first pivot, the middle pivot to the last pivot. This time the arm is bent at 90 degrees at the middle pivot. That’s the only difference. So it’s an extremely simple change to this very well-known mechanism.”


Segerman is a mathematician and an associate math professor at Oklahoma State University. His YouTube channel is filled with years of math and engineering explorations, including 3D-printed models and puzzles.

Learn more with these related lever videos on TKSST:
β€’Β The mighty mathematics of the lever
β€’Β An accidental toy inventor’s shapeshifting designs
β€’Β Angular momentum demo with a Hoberman Sphere
β€’Β The Putter and the art of putting together scissors

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