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

In conclusion, when we put a drinking straw in water, place our finger over the opening and lift it out, the water stays in the straw due to the pressure difference created between the inside and the outside of the straw. The atmospheric pressure outside the straw pushes the water upwards, counteracting the gravitational force pulling the water downwards, leading to a delicate balance that holds the water inside the straw until the finger is removed, and pressure equalizes.

Step-by-step solution

Understand the concept of pressure

Pressure is defined as the force acting perpendicularly on a unit area. In this case, we'll focus on atmospheric pressure, which is the force exerted by the atmosphere on any given point on Earth. Atmospheric pressure at sea level is about 101,325 Pascals (Pa), though it may vary slightly depending on the location and weather.

Identify the factors contributing to the phenomenon

When we put a straw in the water and place our finger over the opening, we are essentially closing off the air inside the straw from the atmospheric pressure. The pressure inside the straw becomes different from the atmospheric pressure outside the straw.

Understand the role of pressure difference

When the straw is lifted out of the water while keeping the finger over the opening, the air pressure inside the straw is less than the atmospheric pressure outside. The water in the straw then creates an upward force, which helps to hold the water inside the straw. This occurs because the pressure difference between the inside and outside of the straw pushes the water upwards, counteracting the gravitational force pulling the water downwards.

The role of gravity

Gravity is responsible for pulling the water downward when the straw is lifted. However, due to the pressure difference, the atmosphere essentially pushes the water upward against gravity, which causes the water to remain in the straw. This delicate balance keeps the water in the straw until the finger is removed, and the pressure equalizes, causing water to flow out of the straw. In conclusion, the phenomenon of water staying in a lifted straw with a finger placed over the opening is due to the pressure difference between the inside and outside of the straw, and the balance between gravitational force and atmospheric pressure.

Key concepts

Pressure Difference

When you cover the top of a straw filled with water and lift it out of the water, you start experiencing the concept of pressure difference. As you block the opening at the top, you effectively isolate the air inside the straw from the outdoor atmospheric pressure. This creates a distinct pressure environment inside the straw compared to outside.
This is crucial because the atmospheric pressure, sitting at about 101,325 Pascals at sea level, is what normally pushes down on all sides equally. However, the air trapped inside the straw can exert less force compared to this outside pressure.

• The pressure inside the straw becomes less than the atmospheric pressure outside.
• This difference results in the water inside creating its own force, holding it in place against human intuition.

The magic lies in how this pressure difference naturally occurs and counteracts the forces such as gravity, making this everyday geyser possible.

Gravity

Gravity is the force that pulls objects towards the Earth. In our straw experiment, it is gravity attempting to drag the water down once the straw is lifted. This is the same force that would pull something out of your hand if you were to let go.
While gravity works persistently to bring the water down, the atmospheric pressure outside the straw pushes the water upwards into the straw.

• This results in a unique tug-of-war between gravity and atmospheric pressure.
• Despite gravity's pull, the pressure difference ensures water stays in the straw.

Thus, while gravity wants everything to fall downwards, it's this balance of forces that lets us observe water defying gravity momentarily.

Force Balance

In the straw scenario, force balance plays a remarkable role. It's all about achieving equilibrium between differing forces. As you cover the straw's opening, the water within is held up by the atmospheric pressure (acting upwards), which counteracts the gravitational force (acting downwards).

• Atmospheric pressure applies an upward force on the bottom of the trapped water.
• Gravity consistently applies a downward force.
• The force balance is achieved when the upward force exactly counterbalances the weight of the column of water.

This delicate balance is disturbed the moment you remove your finger. As air rushes back in, pressures equalize, and the gravity reigns supreme, causing the water to fall. Force balance ensures all sides are in a stalemate until an external force disrupts it.