Q1.
Assertion (A): Compressions and rarefactions move through the medium. Reason (R): Individual particles of the medium continuously move forward with the wave. Choose the correct statement: (i) Both A and R are true, but R is not the correct explanation of A. (ii) Both A and R are true, and R is the correct explanation of A. (iii) A is true, but R is false. (iv) A is false, but R is true.
Answer
(iii) A is true, but R is false.
- A is true. Compressions and rarefactions really do travel forward through the medium — that is what a sound wave is.
- R is false. The individual particles do not move forward with the wave. Each particle only oscillates about its own mean position, parallel to the direction of propagation, and returns there once the wave has passed.
Why the pattern can move while the particles do not: think of the marked turn on the slinky in Activity 10.5. It goes forward a little, comes back, goes forward again — yet the bunched-up region sweeps the whole length of the slinky. Each particle pushes its neighbour and steps back; the neighbour pushes the next one and steps back. The crowding is handed on, but no particle makes the journey. What is transported from the source to your ear is energy, not matter.
Check it yourself: if R were true, standing in front of a loudspeaker would feel like standing in a steady wind blowing out of it. It does not — proof that the air is not flowing towards you.