When a full glass of water covered by a thin piece of cardboard is inverted, the cardboard remains in place. Why does this happen?
1 Answer
There are several forces that are acting on the inverted glass of water and cardboard that keep the water from falling out. First of all, the air pressure outside of the glass is greater than the pressure inside the glass, and greater than the force of gravity acting on the water and the piece of cardboard. You can infer this because the water doesn't fall out, even though the glass is upside down.The lower pressure inside the glass is caused by the fact that gravity pulls the water in the glass downward slightly, increasing the volume of air in the glass. According to Boyle's law, if temperature and the amount of gas are constant, increasing the volume of a gas decreases its pressure. The effect is a net upward force of air pressure acting on the cardboard and glass of water, keeping the cardboard in place and preventing the water from falling out. There is also a force called surface tension that occurs between the water and the cardboard, which prevents the cardboard from detaching from the water.
