Paper lid

Safe­ty pre­cau­tions

Warn­ing! Only un­der adult su­per­vi­sion

Equip­ment

• glass;
• tint­ed wa­ter;
• pa­per or card­board.

Step-by-step in­struc­tions

Fill a glass with tint­ed wa­ter and cov­er it with a piece of pa­per or card­board. Turn the glass up­side down, hold­ing the pa­per lid. Take your hand away from the lid – and the lid stays in place!

Process de­scrip­tion

The pa­per lid is held in place by at­mo­spher­ic pres­sure, which acts on all ob­jects with­in the Earth's at­mos­phere. When the glass is in­vert­ed, grav­i­ta­tion­al forces pull the wa­ter in the glass down­ward. This in­creas­es the vol­ume of the re­main­ing air in the glass, and the air pres­sure in­side the glass be­comes low­er than at­mo­spher­ic pres­sure. More­over, even the to­tal pres­sure of the wa­ter and re­main­ing air com­bined on the pa­per lid is less than at­mo­spher­ic pres­sure, and the lid holds! It's worth not­ing that the wa­ter’s sur­face ten­sion also helps hold the pa­per in place.

Even more cool ex­per­i­ments await you in the MEL Physics sub­scrip­tion!