Equivalence of tension of spring attached to pole with mass vs mass at both ends.

Two apparatus are set up with springs of close to identical spring constant. One has a spring attached to a stationary metal rod at one end and has a weight hanger on the other (Example A). The second has weight hangers at both ends (Example B). The professor can ask students if the springs will have the same tension in them if 500 grams is placed on all three hangers. Once the mass is placed a ruler can be used to demonstrate that the extension of the springs is identical hence, by hooks law, the force on each spring is the same. This is a variant of the thought experiment of a horse pulling on a tree though a rope vs two horses pulling on each other through a rope.
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All this equipment is commonly used. If a measurement is required rather than just judging by extension of spring the spring scales in Le Conte 1-1 can be used instead.