#couple.building_block  something joined by two equal and opposite forces that act along parallel lines
  supertype:  #building_block__unit  a single undivided natural entity occurring in the composition of something else; "units of nucleic acids"
  subtype:  #dipole  a pair of equal and opposite electric charges or magnetic poles separated by a small distance
  part:  #moment.force  the moment of a couple is the product of its force and the distance between its opposing forces

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