AP Physics 1 Question 159: Answer and Explanation
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9. A pipe full of air is closed at one end. A standing wave is produced in the pipe, causing the pipe to sound a note. Which of the following is a correct statement about the wave’s properties at the closed end of the pipe?
- A. The pressure is at a node, but the particle displacement is at an antinode.
- B. The pressure is at an antinode, but the particle displacement is at a node.
- C. The pressure and the particle displacement are both at nodes.
- D. The pressure and the particle displacement are both at antinodes.
Correct Answer: B
B-The closed end of the pipe makes it impossible for the air particles right next to the end to vibrate at all, which is why a particle displacement node must exist there. It's easiest, I think, just to remember that in a sound wave, pressure is at an antinode when particle displacement is at a node and vice versa. Or remember that the pressure must be equal to atmospheric pressure at the open end, which is exposed to the atmosphere, and greater inside because the air is compressed, not sucked out of the pipe.