Air pressure is caused by molecules in the air moving around and colliding with things. We are bombarded by billions of these air molecules at any instant. Since the pressure on all sides of our body is the same, we don’t feel it. At high altitude, where the air is less dense, there are fewer molecules in any given volume of air, and therefore the air pressure is lower.
Have you ever taken a drinking straw, put it in a glass of water, and with your finger over the top of the straw, lifted it out of the water? Try it!!!
The water stays in the straw, and won’t come out until you remove your finger from the top of the straw. Why doesn’t gravity cause the water to fall out?
It all has to do with air pressure. The air is pushing on the water from the bottom of the straw, keeping it in the straw. The water is trying to fall out, but with the top sealed up, the air inside the straw expands when the water moves downward. This lowers the pressure inside the straw, and the air pressure pushing up quickly becomes greater than the air pressure inside the straw, holding the water in the straw. As soon as you move your finger off the top of the straw, the air pressure on both ends becomes equal. The unequal force acting on the water at this point is the force of gravity, and the water falls out of the straw. This is the same thing that happens when you try to dump liquid out of a small necked bottle and it doesn’t go out. If the neck is a little wider, the force of gravity could overcome the air pressure, and the liquid might come out.
The water stays in the straw, and won’t come out until you remove your finger from the top of the straw. Why doesn’t gravity cause the water to fall out?
It all has to do with air pressure. The air is pushing on the water from the bottom of the straw, keeping it in the straw. The water is trying to fall out, but with the top sealed up, the air inside the straw expands when the water moves downward. This lowers the pressure inside the straw, and the air pressure pushing up quickly becomes greater than the air pressure inside the straw, holding the water in the straw. As soon as you move your finger off the top of the straw, the air pressure on both ends becomes equal. The unequal force acting on the water at this point is the force of gravity, and the water falls out of the straw. This is the same thing that happens when you try to dump liquid out of a small necked bottle and it doesn’t go out. If the neck is a little wider, the force of gravity could overcome the air pressure, and the liquid might come out.
