NCERT Solutions for Class 9th Science Chapter 10 Let us observe — Activity 10.5
Book page 188 Updated on2026-09-08
Q1.
Give the slinky at your end a sharp push towards your friend and then quickly pull it back again (Fig. 10.8). Do you observe a disturbance created in the slinky which moves towards your friend?
Answer
Yes. A single bunched-up region — a place where the turns are pressed close together — races along the slinky towards your friend.
Why it happens: your push squeezes the first few turns together. Each squeezed turn presses on the turn ahead of it and then springs back, so the squeeze is handed forward from turn to turn. What moves along the slinky is the pattern of crowding, not the turns themselves — each turn simply moves forward a little and comes back to where it started.
This is a perfect model of a single compression in a sound wave: a push at the source, a crowding relayed forward by collisions, and no bulk flow of the medium.
Q2.
Now, push and pull the slinky end multiple times in quick succession (The pulling and pushing of the end of the slinky is similar to the sound being produced continuously). Are a series of disturbances produced in the slinky? Do these disturbances move across the length of slinky? Does the mark on the slinky move back and forth parallel to the direction of the disturbance?
Answer
Yes to all three.
A series of disturbances is produced. Regions where the turns are close together alternate with regions where they are more spread out.
They travel along the slinky, one after another, from your end to your friend's end.
The marked turn does not travel. It only oscillates to and fro about its rest position, and it does so parallel to the direction in which the disturbance moves.
In the slinky
In a sound wave in air
Turns close together
Compression — density above average
Turns spread apart
Rarefaction — density below average
The marked turn oscillating in place
An air particle oscillating about its mean position
The bunching pattern moving forward
The sound wave travelling forward
Why this makes sound a longitudinal wave: the particles vibrate along the same line the wave travels along. A wave with that property is called a longitudinal wave. Compare a wave you make by shaking a rope up and down: there the particles move perpendicular to the travel direction, and that is a transverse wave (Fig. 10.13).
Tip: the marked turn is the whole point of the activity. It is the visible proof that energy travels while matter stays put.