-- 6 people all trying to push a car out of snow
-- a Tug-o-War with 30 people of different sizes pulling on each end of the rope
-- you and your sister both pulling on the same doll (or Transformer)
-- lifting a book up from the table to a high shelf
taking a book down from a high shelf to the table
(one force is you; another force is gravity)
-- grabbing your big dog by his collar and trying to bring him inside
-- three people at the table all grab the ketchup bottle at the same time