Opening a Jar: Physics Behind Jar Lid's Resistance to Opening

[Gondur]

Hello,
I bought a jar of sauce today.
I tried to open the top, but I couldn't - it wouldn't budge at all.
Then I shook the jar - so the sauce inside moved around.
I tried again and I effortless opened the jar.

Now - this got me thinking, why when I shook the jar did it open easily?
prior to shaking, the jar would have been sitting for days on a shelf.
There was a layer of air between the lid and the sauce.
after shaking, this layer of air mixed with the sauce and was no longer present.
so, perhaps it was the layer of air between the lid and the sauce that made the lid impossible for me to open?

Could someone explain the physics behind this phenomenon I encountered?
Thank you.

 

[sophiecentaur]

Hot hands?

 

[CWatters]

Silly question but did you turn the lid the same way both times?

 

[sophiecentaur]

Perhaps there was a crystallised region between the lid and the top of the jar - or maybe in the beginning of the thread. When you shook up the liquid, it dissolved the crystals and released the stiction.

 

[Tom.G]

why when I shook the jar did it open easily?

I've had the same experience on occasion -- and am still looking for a definitive answer. Any definitive answerers out there to satisfy irrelevant curiousity?

 

[Baluncore]

Any definitive answerers out there to satisfy irrelevant curiousity?

Yes.
It has everything to do with the way you hold the lid. If you apply the grip pressure equally to all fingers and space the fingers equally around the lid it is often very easy to undo the lid because you are not distorting the lid. As you apply increasing torque to the lid, allow a very slight slip to your fingers, so they gradually move around the lid. That will usually find a sweet spot where the lid will release, if not, the cyclic massage will crack the stiction between container and lid.

If you can find an aluminium container with an aluminium screw top lid, you can practice the delicate art of removing lids. Most people will grip the lid at two opposing points and squeeze hard to get a grip, which will lock the lid even if it was not tight and rattled before they started.