Technique
Under most use, a bottle opener functions as a second-class lever: the fulcrum is the far end of the bottle opener, placed on the top of the crown, with the output at the near end of the bottle opener, on the crown edge, between the fulcrum and the hand: in these cases, one pushes up on the lever.
However, one may instead use it as a first-class lever, by placing the near end on the top of the crown, and the far end under the crown edge, then pushing down on the lever (thus the output is on the opposite side of the fulcrum from the hand). This is particularly used with bar blades, which form an obtuse angle. Mechanically, this is a marginally less effective lever, as the effort arm is shorter, but the action of pushing down is marginally anatomically easier.
While most lever-type bottle openers can be used in either configuration, the designed use can be determined if one of the edges is curved, in which case this edge is designed to sit in the middle of the crown, as the curve concentrates pressure, deforming the crown, and a curved edge does not connect with as much of the crown edge, hence being suboptimal and slipping more frequently if used to connect with the crown edge. This difference can be seen in comparing the traditional opener and contemporary bar blade at right.
Read more about this topic: Bottle Opener
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