I don't think the lids are hard to open because they're screwed on tightly, but because there's a vacuum in the jar? (cf. those Age of Enlightenment experiments where large teams of horses couldn't open vacuum-sealed containers.. history doesn't record whether the horses ever stopped pulling in teams and tried just unscrewing them...)
Possibly you could make them easier to open despite the vacuum by making the thead pitch finer - but I suspect that since the threads are cut into glass rather than steel there's a limit to how fine-pitch they can be made and still be robust.
But anyways - why is there a vacuum in the jar? To preserve the jam? Isn't jam a preserve? Like, I thought the whole raison d'etre of jam was as a way to make fruit keep, unrefrigerated, through the winter? Why must we preserve the preserve? Quis custodiet ipsos custodes?
I'm pretty sure that when old ladies from the Women's Institute make jam to sell at the village fête [at a manufacturing cost of £2 per jar.. which they then sell at a cost of 50p a jar.. to raise money for.. old ladies..] they don't vacuum-seal the jars, and their jam seems to keep just fine.
Lastly (probably well-worn common knowledge - apologies if so - but I haven't seen anybody else mention this so I thought I would just in case): To make it easier to unscrew a jar you can increase the coefficient of friction, increase the torque, or (if I'm right about the vacuum thing!) increase the pressure inside the jar to bring it closer to atmospheric pressure:
If your hands are moist or oily, wrapping something like a tea towel or (dry) dishcloth around the lid can increase the coefficient of friction; this also increases the effective diameter of the lid (thus increasing the torque).
A thin rubber sheet or rubber apron would do this even better (but presumably less useful if one opens one's jam in a kitchen and not a sex dungeon). You can get a special tool (that looks a bit like an automotive oil filter strap-wrench) to increase the torque, from kitchenware shops, but having yet another single-purpose kitchen gadget lying around is pretty annoying.
Running the jar under a hot tap would increase the internal pressure (but seems a bit wasteful of energy/water). Piercing the lid with an awl would increase the internal pressure (and an awl seems like a less annoying and more useful thing to have around the house than a jam-jar-strap-wrench-thingy) but is it a problem for shelf-life if the lid has a small hole in it? If jam genuinely is a preserve, possibly not!
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I'm pretty annoyed today, for nominal reasons ranging between ‘petty’ and ‘doesn’t even make sense’. I’m not entirely sure how or if to take oneself seriously when one has such absurd grievances. But that’s a question for another time—I’m here now to tell you about my one potentially valid peeve.
I understand that gender is complicated and difficult, for the whole species (and honestly probably more so for some other species). And it can be hard to tell exactly if anyone is behaving badly regarding it, at least in my modern bubble. Maybe women just aren’t that into designing programming languages? Maybe the thing I’m saying is just boring and a man is saying a more interesting thing?
But a thing that is undeniable is that women want to open jars, dammit! What’s your nuanced explanation there, Bonne Maman? Does the proper amount of friction for maintaining spread safety fall just between the male and female human grip strength distributions?
This study suggests that would be about 400N Fmax (though this would not avert most elite female athletes acquiring jam, see second figure, and the pictured participants are young adults):
The distributions are really surprisingly not-overlapping!
90% of females produced less force than 95% of males. Though female athletes were significantly stronger (444 N) than their untrained female counterparts, this value corresponded to only the 25th percentile of the male subjects.
We know that men and women have different grip strengths! We know that about half of people are women! Why do so many containers require women asking a man for help in order to open them? (Or carrying around an opening tool or living in a kitchen?)
Yes, strength required to open packaging ranges across a wide distribution, but I note that very few are impossible for anyone to open, so it seems like some effort somewhere goes into keeping them in the feasible range, and that effort does not seem to care about it being reliably feasible for people like me. I don’t imagine Bonne Maman wants to stop women getting to their jam—I imagine that nobody cares.
I thought about this most when I lived in a group house with a shared bulk stash of Gatorade, and any time my woman housemate or I wanted to drink a red one we’d have to ask a guy to open it for us. But these days I also often hurt my hands opening (or failing to open) things, and while I’m sure I’m low in the female grip strength distribution—and may also be high on the ‘unreasonable anger about anything nearby when my hands are hurt’ distribution—I don’t think it’s just a me issue, and in the moment it always feels like a ‘fuck you, raspberry jam isn’t meant for you’.