Questions: You put a straw into a glass of water, place your finger over the top so that no air can get in or out, and then lift the straw from the liquid. You find that the straw retains some liquid. How does the air pressure P in the upper part compare to atmospheric pressure PA? greater than atmospheric pressure Equal to atmospheric pressure Less than atmospheric pressure Cannot be determined from the given information

Solution Steps

When a straw is placed in a liquid and the top is sealed with a finger, the liquid remains in the straw when lifted. This phenomenon is due to the difference in pressure between the inside of the straw and the atmospheric pressure outside.

The pressure inside the straw, denoted as \( P \), is less than the atmospheric pressure \( P_A \). This pressure difference is what prevents the liquid from falling out of the straw. The atmospheric pressure outside the straw is greater, pushing the liquid up and keeping it inside the straw.

Based on the principles of air pressure and fluid dynamics, the air pressure inside the straw is less than the atmospheric pressure outside. This can be expressed mathematically as: \[ P < P_A \]

Final Answer