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I have a pot with a lid that has a rubber seal (not a screw cap). When taking the lid off, it is incredibly difficult to lift the lid by pulling straight up on it, but twisting the lid whilst pulling gently upwards, as many people would do instinctively, makes it easy. What about this act of rotation makes it easier to take the lid off?

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  • $\begingroup$ Are you familiar with static vs. kinetic friction? $\endgroup$ – Aaron Stevens Jul 27 at 20:56
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    $\begingroup$ @AaronStevens I wasn't familiar with the distinction between those two when I asked the question – it seems that was the issue that prevented me from working it out $\endgroup$ – Josh Jul 28 at 15:41
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What about this act of rotation makes it easier to take the lid off?

I don't disagree with @niels nielsen answer, but would like to offer a possible alternative explanation.

In order to get the lid off you need to exert a force parallel to the lid/seal interface that exceeds the maximum static friction force between the lid and the seal. Once it starts moving the friction force is reduced to the kinetic friction force, making it easier to pull the lid up and remove it.

The amount of force you need to apply parallel to the lid/seal interface to overcome static friction is less when you twist the lid than when you simply pull up on the lid. That's because you are taking advantage of torque when you twist the lid. Torque is the rotational analog of force. The torque is equal to the force you apply parallel to the lid/seal interface times the radius of the lid if you are grasping the perimeter of the lid. The greater the radius, the less the force that needs to be applied to achieve the same amount of torque needed overcome static friction. In fact if you were able to use, in addition, a wrench to loosen the lid it would be even easier since the wrench would increase the moment arm further reducing the force you need to apply for the same torque. It's somewhat analogous to using a wrench to free a nut from a rusted bolt, than simply doing it by hand. It is considerably easier using the wrench.

Also, depending on the nature of the seal lid interface, only pulling up on the lid without twisting may result in compressing, or jamming, the rubber seal between the pot and the lid, making it even more difficult to remove.

Hope this helps.

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  • $\begingroup$ I mostly agree with this. But this part: "The amount of force you need to apply parallel to the lid/seal interface to overcome static friction is less when you twist the lid than when you simply pull up on the lid. That's because you are taking advantage of torque when you twist the lid." I think needs a stronger argument. You say torque is stronger for a force applied at a larger radius, but that doesn't really explain why in this case the rotation is still better than just pulling up. Are you saying for a smaller lid you wouldn't want to twist? $\endgroup$ – Aaron Stevens Jul 27 at 22:20
  • $\begingroup$ @AaronStevens I confess I was thinking more in terms of freeing the lid from a jar with a rubber seal where you grasp the perimeter of the lid. The top knob of a pot is likely to have a much smaller radius than the radius of the lid, in which case the advantage of torque when grasping the knob of the pot would be much less than when grasping the perimeter of the lid. As always, thanks for the insightful input. $\endgroup$ – Bob D Jul 27 at 22:29
  • $\begingroup$ Yeah I figured you were talking about grabbing from the ends. I wasn't asking about lids with a knob. $\endgroup$ – Aaron Stevens Jul 27 at 22:35
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When the lid to the pot is "glued on" and you are pulling straight up, you are trying to break the bond in tension all the way around its circumference all at the same time, which is hard to do.

But since the glue joint is weaker in shear than in tension, and since twisting the joint places the glue joint in shear, you have a better chance of breaking the joint while twisting.

When you combine tension with shear, you create the opportunity for the weakest part of the glue joint to debond locally and when it does, the applied stresses in the remainder of the circumferential joint are increased- especially right at the debonded spot, which concentrates the stresses- and the joint "unzips" incrementally (but quickly) all the way around the lid.

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    $\begingroup$ A few other thoughts. Isn't there also the advantage of applying a torque to overcome the maximum static friction force between the lid and seal than simply pulling up on the lid? Also, depending on the seal/lid interface, only pulling up on the lid could compress, or jamb, the seal making it more difficult. $\endgroup$ – Bob D Jul 27 at 21:57
  • $\begingroup$ sounds like a fun series of experiments to perform! $\endgroup$ – niels nielsen Jul 27 at 22:37
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    $\begingroup$ Amen to that. Who would of thought that taking the lid off a pot could generate so much scientific interest! $\endgroup$ – Bob D Jul 27 at 22:39
  • $\begingroup$ Thanks very much for the answer. What does it mean to say "the glue joint is weaker in shear than in tension", and why is this the case? $\endgroup$ – Josh Jul 28 at 15:57
  • $\begingroup$ this is generally true of glued joints- this is what it means: imagine pressing your hands together in front of you with glue between them. now you're stuck. push one hand away from you and pull the other towards you, as if you wanted to rub your hands together. this action places the glue joint in a state of shear instead of tension, where you are trying to simply pull your hands apart. $\endgroup$ – niels nielsen Jul 28 at 16:47

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