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Can water defy gravity?
Try this..
Suck some water up a straw and then put your thumb over the end that is in your mouth. Now keeping your thumb over the straw take it out of the water. What happens?
Now take your thumb off the end. What happens?
Photo of a glass of water with a straw in
You will need:
- A glass
- Water
- A straw
What's happening?
You should have found that when you took the straw out of the water, the water that you’d sucked up stayed in the straw. When you took your thumb off the end of the straw, the water came out of the bottom of the straw.
This experiment is all about the forces acting on the liquid. One of these forces is gravity, the force which pulls all objects towards the centre of the Earth. Gravity is pulling the water in the straw down towards the Earth, so why doesn’t the water fall out of the straw when your thumb is on top?
There must be another force, stronger than gravity, pushing upwards on the water. This force comes from air pressure. Air pressure is caused by molecules of air pushing against things. Air pressure is very strong and very important. It affects the weather, and the weight of all the air above us in the atmosphere pushing down is a very large force.
When the straw is just sitting in the glass there is nothing separating the air in the atmosphere from the air in the straw. This means that the air in the atmosphere and the air in the straw are pushing down on the water in the glass with the same force. When you suck on the straw it makes the water move up the straw. If you put your thumb over the end it traps the water in the straw, and your thumb separates the water in the straw from the air pressure of the atmosphere. If you pull the straw out of the water and keep your thumb over the end, the water stays in the straw. This is because there is no air pushing the water down from the top of the straw where your thumb is, but the air in the atmosphere is still coming up the open end at the bottom of the straw and pushing up against the water to keep it in the straw. The force from the air in the atmosphere pushing up is stronger than gravity pulling down! If you remove your thumb from the end of the straw the water will flow back out. This is because without your thumb there, the air is pushing with the same force from both ends of the straw. These two pushes cancel each other out so that gravity can pull the water down to the Earth, just as it was trying to do all along!
