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?
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.
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.