The mechanics behind opening a beer

Have you ever opened a beer with a lighter (or something different from a bottle opener)? I surely have! Once I was trying to teach someone how to do it and I was struggling to understand why this person was failing to accomplish it.

Well, after a while I just said: “tighten more and you will do it”. Job done, the bottle was opened. To explain why this works I created this simple image capturing (my idea of) the statics behind this phenomenon.

My personal interpretation of the phenomenon: the lighter can be considered a rigid beam pinned in correspondence of the bottle cap. The hand provides a flexible support to the lighter. Both supports allow to counterbalance the moment couple m coming from the hand that holds the lighter.

Let’s say that you need a force F_max in correspondence of the bottle cap to open it. You can increase the rotation of the lighter to increase the couple and, in turn, the reaction of the support, which must equal F_max for the bottle to open. For a given rotation of the lighter, the amount of the couple m is dependent on the stiffness of the flexible support (k).

My idea is that by tightening the hand that holds the bottle, you activate the muscles of your fingers, thus increasing the stiffness k. Therefore, if you tighten enough, you will be able to open the bottle. Please leave a comment if you have a different idea and want to talk more about this.