Why does tapping a jar lid make it open more easily?
This question comes from Summer at Outside Kid and her kids. It would take some more involved experimenting than I can do to be sure, but here's my best guess. Thanks to my husband for helping me with this answer.
First, let's think about what forces are working against you as you unscrew the lid. Friction is the force between two surfaces that opposes motion between surfaces. The magnitude of the friction is dependent both on the types of materials involved, and the surface area in contact between the surfaces.
The other important force is the one created by the low pressure inside the jar. The manufacturer intentionally removes air from the jar so that more particles push on the lid from the top than from the bottom. The lid is pinned to the jar by the weight of the air molecules pushing down on it.
Now, back to the knife tap. The knife tap bends the lid just a little bit. This decreases the surface area in contact with the jar, and may let in some air. The decreased surface area leads to a decrease in the magnitude of the friction. Any air that comes into the jar, helps push the lid up. For more on why air would flow into the jar, read about playing with a straw here.
Thank you, Summer, for this great question! I am always amazed at how complicated every day physical events really are. Summer's family has been busy building the coolest tree house ever! You might want to click over and take a peek.
The goal of this Science Along the Way Series is to help me, my kids, and my readers appreciate the wonder of the physical world that surrounds us every day. If you find an phenomena worth pondering along your way, I would love to feature it as part of this series. Guest posts and questions are welcome. E-mail me at WisdomKnowledgeJoy@gmail.com.