Solution to Activity 11.4 of NCERT Class 9 Science Chapter 11 Work and Energy

4. Complete Activity 11.4 (Page 129).

• Take a slinky. Ask your friend to hold one end. You hold the other end. Now stretch the slinky as shown in Fig. 11.5(a). Then give it a sharp push towards your friend.

•What do you notice? If you move your hand pushing and pulling the slinky alternatively, what will you observe?

• If you mark a dot on the slinky, you will observe that the dot on the slinky will move back and forth parallel to the direction of the propagation of the disturbance.

Answer:

Aim: To alternately push and pull the slinky and conclude based on what you observe.

Materials Required: Slinky.

Procedure:

(i) Ask you friend to hold one end of the slinky while you hold the other end.

(ii) Now stretch the slinky as shown in (A).

(iii) Now alternately push and pull the slinky and note the observations.

(iv) Mark a dot on the slinky and repeat the activity. Observe what happens to the dot.

Observations:

When the slinky is alternately pushed and pulled, there are regions where the coils become closer called compressions (C) and there are regions where the coils are further apart called rarefactions (R).
The dot marked on the slinky moves back and forth parallel to the direction of the disturbance.

**Sound Wave Model:** Do you see the alternate compressions (C) and rarefactions (R) in the slinky? This is exactly how a sound wave propagates.

Conclusions:

The slinky is a model for the propagation of sound in a medium.
In sound waves the individual particles of the medium move in a direction parallel to the direction of propagation of the disturbance (as indicated by the dot in the activity). The particles do not move from one place to another but they simply oscillate back and forth about their position of rest. Hence sound waves are called longitudinal waves.

“Take a slinky. Ask your friend to hold one end. You hold the other end. Now stretch the slinky as shown in Fig. 11.5(a). Then give it a sharp push towards your friend.

•What do you notice? If you move your hand pushing and pulling the slinky alternatively, what will you observe?

If you mark a dot on the slinky, you will observe that the dot on the slinky will move back and forth parallel to the direction of the propagation of the disturbance.” – Solved.