Exercise 10.4
Give the slinky at your end a sharp push towards your friend and then quickly pull it back again (Fig. ). Do you observe a disturbance created in the slinky which moves towards your friend?
Not cross-checked
NCERT prints no numerical answer for this exercise, so this working has not been cross-checked against the book.
NCERT’s answer
(iii)
(iii) A is true, but R is false.
A is true: compressions and rarefactions — the regions of higher and lower density — really do travel forward through the medium, each particle passing the disturbance to the next by collision.
R is false: the particles themselves do not move forward with the wave; each only oscillates to and fro about its own mean position, parallel to the direction of propagation.
The slinky shows it directly: the marked turn stays where it is and merely oscillates, while the closely spaced and spread-out regions run the whole length of the slinky.
What travels forward is the disturbance and the energy it carries, not the matter of the medium.