How to distinguish among structures and mechanisms

Distinguishing a structure (left) from a mechanism (right)

A structure is something that has no moving parts. It can move only if all the parts move in the same way – by the same amount and in the same direction. The diagram on the left shows one way to make a structure. Each fastener goes through both strips and through the base. Whenever a strip is attached to the base at two points it cannot move with respect to the base. Thus the strip and the base are a structure.

A mechanism is a set of attached pieces that has at least one moving part, which can move while another part stays still.  The diagram on the right shows one kind of mechanism.  Each of the independent strips is attached to the base by its own fixed pivot.  Fixed pivots are represented by solid circles.  Each strip can turn around with respect to the base so each one is a mechanism.


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