Do you know that water can defy gravity and float in midair?
It sure can. Let’s try this magical floating water experiment.
What you need
- clear plastic cup
- index card or thick piece of paper
- empty bucket
- Fill the cup to 3/4 full with water.
- Cover the cup with the index card. Make sure it completely covers the cup’s month.
- Keep the card in place by pressing on it with one hand and move the cup to above the bucket with the other hand. The bucket can catch the water in case you don’t succeed on your first try.
- Slowly turn the cup upside down while pressing firmly on the card to prevent water from leaking.
- Take a deep breath and remove your hand from the card. There you go, magical floating water!
Air molecules, which we cannot see with naked eyes, are constantly moving and bombarding everywhere.
The resulting force is air pressure.
Normally, the air pressure above and below the water are the same.
When equal forces are applied in opposite directions, they cancel each other out and then gravity takes over to pull the water down.
That is why if you invert the cup with water only (no index card), the water just pours out.
When you cover the cup with an index card and hold the card in place while turning the cup upside down, your hand applies an upward force to prevent the card and the water from falling.
When your hand covering the card is removed, the card and the water don’t drop.
This is because despite losing the upward force from your hand, several new forces come into play.
First, water molecules are attracted to the card and the cup (adhesion) exerting a tiny force to hold the whole system together.
Second, water molecules are also attracted to each other (cohesion).
Together, these two properties of water create an airtight seal around the cup’s rim, allowing the volume occupied by the air inside to increases under gravity.
Imagine the same amount of air molecules bombarding a bigger space.
As a result, the air pressure inside decreases. Now the air pressure outside the cup is bigger and holds the card and the water in place.
If you let the water leak from the crack or the outside air seep into the cup, even a tiny bit, the seal will be broken.
The air outside will enter the cup to equalize the pressure inside. All the water and the card will fall (into the bucket if you have followed the steps above). 🙂
Here are more fascinating experiments making use of air pressure:
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