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Am I correct in assuming it is the weight of all that liquid + gravity?

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Why am I opening upside-down? My hand is hurt :( –  Adel Jul 14 '11 at 22:35
    
What kind of lid does the bottle have? –  sharptooth Jul 15 '11 at 8:06
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I know that if one tries to unscrew a bottle by turning the body and holding the cap fixed it opens more easily. The reason has to do with angular momentum. Were you maybe turning the body and keeping the cap fixed? It is a multiparameter problem then, like climate :) . –  anna v Jul 15 '11 at 10:56
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@Georg if you are talking to me, try it. I have very weak finger grip and often I have to resort to this trick to open screw tops. you get RXp on the whole bottle compared to rXp of the small cap, for the same turns. Or you can think of it as a lever. –  anna v Jul 15 '11 at 19:47
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@Adel you did not reply if when it is up side down you turn the cap, or turn the body of the bottle keeping the cap steady. In the latter case it is the angular momentum that does it, nothing to do with gravity –  anna v Jul 16 '11 at 12:13

2 Answers 2

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The weight of liquid impact will be minimal (since with a screw cap the force will be redirected along the screw threading) - it's the lubricating effect of the liquid that actually helps. You start unscrewing, the liquid gets into the threading and lubricates it.

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The weight is caused by gravity. So, yes, gravity works for you if you turn the bottle upside-down.

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If the soda bottle has a positive pressure (which it does), doesn't the weight of the fluid work to increase the differential over the cap/mouth, thereby working against you? –  AlanSE Jul 16 '11 at 0:15
    
Remember the plastic cap is two parts - the one you twist and a ring. Then there's a spiraling little ridge... so I do believe it is easier to open. :) –  Adel Jul 16 '11 at 5:21

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