Answer to Question #274740 in Astronomy | Astrophysics for Lisa

Question

Fill a 250 mL beaker with water. Get two plastic straws 12 cm long. Place one straw

in the beaker with water. Place the second straw in the position shown in the figure, and blow

hard. Ask a classmate to observe the level of water in the first straw before blowing and while

blowing into the second straw. Why does the water rise when you blow into the second straw?

Answer 1

Explanations & Calculations

• When there is no airflow just above the straw pressure there is equal to that at the water surface.
• So no pressure difference across the straw inside it.

• This does not cause any force on the water column inside the straw hence it remains level at the same level as the surrounding water.

• But when there is a sufficient flow of air just above the straw, there is a pressure difference generated inside the straw causing the water to be lifted compared to the surrounding waters.

• When airflow is in action, the air has a speed. According to Bernoulli's law;$\small P+1/2\rho v^2+\rho gh = k$, pressure at that area drops compared to where there is no flow.

• As soon as a pressure drop occurs, a pressure difference is generated. According to the relationship; $\small \text{Pressure}=\text{Force}/\text{Area}$ , this difference creates a force on the inside water column.

• This phenomenon lifts the water column provided that the airflow is strong enough to drop the pressure so that enough pressure difference is generated.