Why does the water stays in the straw when you cover the top of the straw with y

Question

Why does the water stays in the straw when you cover the top of the straw with your findger?

Also, can someones help answer the following questions!

a. What is the net force on the water in the straw?

b. Draw a free body diagram for a system that is the water.

c. Is the gas pressure inside the straw, between the liquid and your finger, greater than, less than, or equal to atmospheric pressure? Explain.

d. If your answer to part c was “greater” or “less,” how did the pressure change from the atmospheric pressure to the final pressure?

Explanation / Answer

When you cover the top of the straw, you block the air from pushing down on the water. In this case, the only forces on the water are the air pressure pushing up, and gravity pulling down. However, the air pressure is a greater force than gravity, and keeps the water in the straw despite gravity's pull.

Air has weight, so gravity tries to pull it down to the ground. The air that is already down on the ground is in the way of the rest of the air, so the rest of the air stays above our heads. The important thing is that gravity pulls air down equally everywhere you look. That means that the of weight of air on the water outside the straw is the same as the weight of air on the water inside the straw. The level of the water in the glass is almost the same everywhere, because the weight of the air is the same everywhere.

a)

When you suck on a straw, you expand your lungs, which reduces the air pressure inside your mouth to less than atmospheric pressure. Then the atmospheric pressure pushing on the liquid in the glass provides a net upward force on the liquid in the straw sufficient to push the liquid up the straw.

c)

The atmospheric pressure pushing on the liquid in the glass provides a net upward force on the liquid in the straw sufficient to push the liquid up the straw. Therefore the atmosphereic pressure is equal to the the gas pressure inside the straw.

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