Egg into a Bottle

Perhaps the most intriguing physics-in-the-kitchen demonstration for all ages is getting a hard-boiled egg with the shell removed into a bottle that has an opening diameter smaller than the minimum diameter of the egg. One solution is to very carefully drop some bits of burning paper into the upright bottle and then place the egg at the opening. Soon, if the sequence is done with the correct timing, the egg will have the urge to go inside. What is the correct timing, and why does the egg have this urge?

Burning paper

Answer

Newton’s second law explains the result. When the egg is resting on the bottle, the ambient air in the room plus the gravitational force on the egg by the Earth together exert a total downward force on the egg that is equal to the total upward force provided by the contact force of the bottle plus the force of the air inside. By    Newton’s second law, a downward acceleration begins when there is a net force downward. To get the egg to accelerate downward, you must reduce the upward force of the air inside the bottle. This action reduces the total upward force, allowing a net force downward, with the resulting acceleration downward dictated by net force = mass times acceleration.

The correct timing requires one to wait until the paper dropped inside the bottle has stopped burning, then immediately and carefully place the egg on the opening. The warmed air inside the bottle will begin to cool just as the burning finishes. The egg seals the opening, so the air pressure inside decreases as the cooling progresses. The total downward force will eventually be greater than the total of the forces resisting the egg at the entrance, and the net force downward will accelerate the egg into the bottle. The movement continues until the egg drops to the bottom. Kerplop!