How does the button on my pickle jar pop once opened for the first time?

Question

I have this button on a few jars around the house like jam jars, they are always glass. Why does the “button” pop once and never again?

In: Physics

Answer 1

When the jar is sealed, there is a vacuum on the inside. This means the pressure outside the jar (air in the room) is greater than the pressure on the inside. The outside air exerts a force on the lid, just as if your finger would when you try to hold down the poppy thing. When you open that jar, air rushes in and the poppy thing is raised, because the pressure has equalized. Afterward, putting the lid back on just traps the air with the same pressure inside, so there is no force pushing down on the poppy thing.