Question

If you put a drinking straw in water, place your finger over the opening, and lift the straw out of the water, some water stays in the straw. Explain.

Short answer

When you lift a drinking straw out of water with your finger covering the opening, the water stays in the straw due to the atmospheric pressure acting on the water surface outside the straw and the low pressure inside the sealed straw. The difference in pressure creates a net force that holds the water inside the straw, countering the force of gravity. This phenomenon is also observed in barometers, which measure atmospheric pressure using similar principles.

Step-by-step solution

Introduce the phenomenon

When you put a drinking straw in water and then place your finger over the opening, some water stays in the straw as you lift it out of the water. This occurs because of the atmospheric pressure acting on the surface of the liquid.

Discuss atmospheric pressure

Atmospheric pressure is the force per unit area that the atmosphere exerts on any surface due to the weight of the air above it. At sea level, the atmospheric pressure is about 101,325 Pa (Pascals). When you lift the straw out of the water, the atmospheric pressure acts on the water surface outside the straw.

Explain the role of the finger

When you place your finger over the opening of the straw and lift it, you are preventing air from flowing into the straw. Since no air is entering the straw, the pressure remains low inside the straw compared to the atmospheric pressure outside.

Understand the difference in pressure

Due to the low pressure inside the straw and the higher atmospheric pressure acting on the water surface outside the straw, there is a net force acting upward on the water in the straw. This force is what holds the water inside the straw, countering the force of gravity, which tries to pull the water downward.

Relate the concept to other situations

This phenomenon can also be observed in a barometer, which is an instrument used to measure atmospheric pressure. It uses the same principle of pressure differences between the inside and outside of a closed container to measure pressure changes. In conclusion, when you lift a straw out of the water with your finger covering the opening, the atmospheric pressure acting on the water surface outside the straw and the low pressure inside the sealed straw create a net force that holds the water inside the straw, countering gravity.