Cap conundrum
Why is it that you can screw on a lid with a certain amount of force, yet you're completely unable to later open said lid?
I've encountered this like ten times over the last few days with pop (midwest baby) bottles and my collection of coffee mugs, and I just know without my readers help I'll end up spending hours googling on it.
Posted by drunkenbatmanFebruary 25, 2005, at 06:29 PM
Comments (13)
Posted by: Joe at February 25, 2005 06:45 PM
When closing a lid, it becomes harder and harder as you twist (more friction) until you're satisfied. The friction you encounter in this process is dynamic, meaning it's the effort required to keep the lid moving. However, when opening the lid you encounter a second type of friction, static friction. This relates to the force required to begin motion of an object. Static friction is generally greater than dynamic friction.
I don't really know why static is greater than dynamic, but I think it's similar to why it's harder to begin pushing a car (e.g. from behind) from a stop versus once the car is in motion.
Posted by: mindflayer at February 25, 2005 07:27 PM
You may also run into a cap that has shrunk - even ever so slightly - in the colder air of a refrigerator. Glass does not contract as much, so there ya go.
Suggestion - work out, or find a stronger woman that you. :D
Posted by: Brett Nordquist at February 25, 2005 07:30 PM
Had this same problem with baby food jars. End up just getting frustrated and tossing them out instead of smacking them against the counter.
Posted by: Jonathan at February 25, 2005 08:09 PM
"I don't really know why static is greater than dynamic, but I think it's similar to why it's harder to begin pushing a car (e.g. from behind) from a stop versus once the car is in motion."
I'm no physicist, but my guess is "inertia." It's easier to keep something in motion once it already is than it is to put it in motion from a rest state.
Posted by: Joe at February 25, 2005 09:12 PM
that seems pretty reasonable. also worth considering is that when in motion, adding force will always result in an acceleration but when the lid is on tight (in a static condition), you can add force for as long as you want but if that amount isn't enough to overcome static friction, the lid isn't gonna budge.
i don't spend much (any) time in garages, but those impact wrenches mechanics use may be related to this question as well (particularly, solving the problem of the stuck lid). instead of clamping onto a bolt (like those on tires) and trying to rotate from a stop, i think the impact wrenches hit the sides of the bolt rapidly. i may be talking about two different concepts here. i'm getting a little too rambly here though so i'll stop.
Posted by: Ben Donley at February 25, 2005 11:38 PM
It's not inertia. It's that when two objects in contact are not in motion, their atoms are able to come closer together, and form stronger bonds. That's why when something sticky is beginning to unstick, you can press on it and stick it down better.
The problem with jars is usually expansion & contraction, and sticky, dried pop all up in your cap.
Posted by: Neil at February 26, 2005 12:06 AM
As above,at a basic level, friction is mostly related to things sticking together. Once you break free, the friction goes way down, but stays constant, not dependent on speed or acceleration.
Inertia is a different game. "A body in motion..." You have to push a car hard (F=M x a, lots of M) to get up to speed, but then, except for a little rolling friction, she just keeps rolling. An impact wrench is pretty much the same thing, a moving weight slaps the shaft. For big bolts, three and four inch diameter, you use a slugging wrench, A two foot hand and a twenty pound sledge hammmer. Inertia
When I run into a jar, usually new and vacuum packed, that woun't open, I use a church key. Pointy end down in the middle of the lid and a smart tap with the heel of your hand makes only a small hole, enough to equalize pressure. This works for baby food jars. Not much help on coffee mugs or bottle tops though. That's a friction problem.
Suggestion: Don't screw them down so hard. There is a lot of mechanical advantage in a screw cap, the threads are tight once you feel the resistance go up. If you get it as tight as you can, against moving dynamic friction, the greater breakaway, dynamicfriction will get you when you try to get it off. You're no stronger either way, but the friction is by quite a lot.
Think getting a lug nut off after the guy at the garage has tighten it with an impact wrench.
Posted by: Adam Flanczewski at February 26, 2005 05:40 AM
Want to know the secret to opening lids?
Mouse pads.
Use a mouse pad. Just grab a nice (real, none of those slick plastic crap ones) and just grip the lid with it, and give it a twist. The rubber used on the bottom of mouse pads is the exact same as those rubber "lib openers" that used to be on the Home Shopping Network. :P
I've done it for years... it never fails. Hope this tip helps.
Posted by: Neil at February 26, 2005 06:48 AM
It's the fungus! Put a cap full of Drano in the jar before you close it and three weeks later it'll re-open easily.
Oops, just spotted your plea, "And do endeavor to appear sane." Hey, ho, maybe next time.
Posted by: Nick at February 26, 2005 11:26 AM
it's partly overcoming static friction, and the problem is usually worst with 2-liter soda bottles (i'm in the midwest and i don't say pop...) that have been in the fridge, so i think the cap shrinks at a greater rate than the bottle in the cold.
Posted by: Jason Terhorst at February 26, 2005 04:14 PM
I'm living in the midwest, but I use "pop" and "soda" interchangably. It depends on my mood. And the fact that it's difficult to say "Pepsi Pop Bottle Top Promo" 3 times fast. Too many "P"s.
Posted by: wicak at February 26, 2005 10:43 PM
one more possibility:
when you are turning the cap, you are overcoming dynamic friction, and together with your warm hands you may be heating up the cap and therefore causing it to expand.
once you are done closing the bottle/container, it cools down and shrinks, causing it to clamp down harder on the bottle.
cheers.
Uhm... pressure changes? I know that's a big one on the booze I drink.