CBSE Class 9 Science Practical Skills – Velocity of a Pulse in Slinky

AIM
To determine the velocity of a pulse propagated through a stretched string/slinky.

APPARATUS REQUIRED
A stopwatch, a slinky (helical spring/string) and a metre scale.

THEORY
A pulse is a single disturbance, suddenly created, moving through a medium from free end to other end (which may he free or fixed) for a while. The distance travelled by the pulse in a unit time is called pulse velocity. Therefore,

Pulse velocity (v) = $$\frac { Distance\quad travelled\quad by\quad a\quad pulse\quad (s) }{ Tie\quad taken\quad (t) }$$     or   v= $$\frac { s }{ t }$$ m/s

PROCEDURE

1. Tie one end of a spring/string with a rigid support and measure its length. Let it be s.
2. Hold the free end of the string/slinky and create a disturbance up and down vertically, in case of transverse wave [Figure 5.3] or vibrate in a back and forth manner in case of longitudinal wave [Figure 5.4],
3. A pulse will be formed and it will travel towards the fixed end.
4. Introduce a wave into the slinky by creating a large number of pulse at regular intervals.
5. Stall the stopwatch at the instant when you create the single disturbance to the first coil and stop the stopwatch when last pulse reaches the fixed support.
6. Record the time (t) taken by the pulse to travel along the slinky of length s.
7. Repeat your experiment for 4-5 times, note the time taken ‘t’ by varying the number of pulse and record your observations.

OBSERVATIONS

1. Length of slinky =…….. cm
= ……….m
2. Least count of stopwatch = ……….s

RESULT
The velocity of a pulse propagated through a stretched string/slinky = ………….  ms-1

PRECAUTIONS

1. One end of the string/slinky should be fixed properly with the rigid support.
2. The string/slinky should be massless, flexible and of proper length.
3. Special attention should be paid while recording the time because variations of time may vary the velocity of pulse to a higher level.
4. Interval between the successive pulse should be maintained.

INTERACTIVE SESSION

Examiner :
What is the aim of your experiment?
Examinee:
To determine the velocity of pulse propagated through a stretched string/slinky.

Examiner :
What is pulse?
Examinee:
It is a single disturbance moving through a medium from one end to another end.

Examiner :
What is the position of each turn of a slinky when it is stretched?
Examinee:
When a slinky is stretched, the individual turn of the slinky to be assumed in an equilibrium or rest position.

Examiner :
While giving the jerk to the free end of a spring, which physical quantity does move to the other end?
Examinee:
Energy moves from one end to other end.

Examiner :
Do the particles of medium also move from one end to the other end?
Examinee:
No, particles of the medium oscillate about their mean position. In the case of transverse wave, they vibrate up and down while in the case of longitudinal wave, they vibrate to and fro.

Examiner :
Suppose, vibration of the coils of a spring are in the same direction in which the pulse propagate.
Which type of pulse is generated?
Examinee:
Longitudinal pulse.

Examiner :
Name a common device available in the laboratory for the production of sound waves in air.
Examinee:
Tuning fork.

Examiner :
Name a device available in the laboratory for studying waves.
Examinee:

Examiner :
Which type of wave can be produced on a slinky?
Examinee:
Both, transverse wave as well as longitudinal wave.

Examiner :
Which type of wave is produced when a stone is dropped on the surface of water in a pond?
Examinee:
Transverse wave.

Examiner :
Examinee:
It is a long helical spring, usually made of steel, flexible and has a elasticity.

Examiner :
Are the waves travelling on a string/slinky mechanical waves?
Examinee:
Yes, the wave travelling on a slinky are mechanical waves.

Examiner :
What will happen when the incident pulse reaches the fixed end of the string/slinky?
Examinee:
Pulse returns to the left after bouncing off and is known as reflected pulse.

Examiner :
If a crest is incident towards a fixed end, the reflected pulse will be crest or trough?
Examinee:The pulse will reflect as a trough.

Examiner :
What is the speed of reflected pulse?
Examinee:
It is same as the speed of the incident pulse.

Examiner :
This means two pulses which travel in different directions will have the same speed.
Examinee:
Yes.

Examiner :
What is the wavelength of incident and reflected pulse?
Examinee:
Both have the same wavelength.

Examiner :
What is the relationship between the pulse velocity (v), wavelength (λ) and frequency (v)?
Examinee:
v = vλ .

NCERT LAB MANUAL QUESTIONS

Question 1:
What is the difference between a pulse and a wave?

Question 2:
State the nature of pulse generated in a stretched string. Is it transverse or longitudinal? Can a longitudinal pulse be generated in a string or a thread (an amateur’s telephone !)?
The nature of a pulse generated in a stretched string is transverse in nature. Yes, longitudinal pulse can also be generated in a string or a thread.

Question 3:
Why we prefer a longer string to perform the experiment?
The speed of the pulse generated in the stretched string/slinky is very high. So, it is difficult to measure the time taken by the pulse in a single journey along the known small length of string. Therefore, it is preferred to take a longer string to perform the experiment.

Question 4:
In this experiment, you must have noticed that while expressing the result for the speed of a pulse in a string, it is not suggested to take the average value of speeds determined with different lengths of the string. Why?
The speed of a pulse ( or a wave) through a stretched string is inversely proportional to the square root of the mass per unit length of the string. Therefore, the different values of speed of the pulse for different values of string length will be observed. Hence we cannot take the average value of speed determined with different lengths of string.

PRACTICAL BASED QUESTIONS
Multiple Choice Questions/VSA

Question 1:
The physical quantity that will remain unchanged after reflection of a sound wave is
(a) velocity
(b) wavelength
(c) frequency
(d) all of these

Question 2:
In air, at room temperature, the speed of sound is 345 m/s. If a tuning fork produces a pure tone
of 1 kHz, the wavelength of sound produced in air is:
(a) 345 m
(b) 0.345 m
(c) 3.45 m
(d) 34.5 m

Question 3:
On which of the following factor, the speed of propagation of pulse in a slinky does not depend?
(c) Room temperature

Question 4:
When a pulse is sent through a slinky/string, the physical quantity that travels along its length is
(a) speed
(b) frequency
(c) wavelength
(d) energy

Question 5:
To find velocity of the pulse in a string, we need
(а) only a measuring scale
(b) only a stopwatch
(c) both measuring scale and stopwatch
(d) neither measuring scale nor stop watch

Question 6:
The distance between compression and rarefaction of a sound wave is 5 cm. Its wavelength is
(a) 5 cm
(b) 10 cm
(c) 2.5 cm
(d) 20 cm

Question 7:
The distance between mid-points of two consecutive compressions or rarefactions in a wmve is given by
(a) 2λ
(b) λ /2
(c) λ /4
(d) λ

Question 8:
If the pulse hits at the fixed end as shown in the diagram A, then immediately reflected pulse is

(a) (i)
(b) (ii)
(c) (iii)
(d) (iv)

Question 9:
What is the velocity of pulse for the slinky indicated in the figure below?

Given that the pulse took 5 seconds to travel from A to B and then back to A.
(a) 5 m/s
(b) 4 m/s
(c) 10 m/s
(d) none.

Question 10:
A slinky, 5 m long, is tied to the hook in a wall and its other end is held tightly. It is then pulled as shown, and released suddenly.

The disturbance created on the slinky is
(a) Transverse wave
(b) Longitudinal wave
(c) Longitudinal pulse
(d) Transverse pulse

Question 11:
The wave pulse can be produced on a rope whose one end is fixed
(a) by giving a single jerk to the other end
(b) by giving more jerks to the other end
(c) by giving continuous jerks
(d) either (a) or (b) or (c)

Question 12:
Shyamal was calculating the velocity of wave using a slinky. He asked his teacher regarding the features of spring to be used. The teacher replied that the spring should be
(a) long, soft and flexible
(b) short, soft and flexible
(c) short, hard and flexible
(d) long, soft but not flexible

Question 13:
In the experiment for determining the velocity of a pulse propagating along the length of string, we prefer a long thick cotton string
(a) because pulse cannot be formed in a thin short string
(b) because cotton string is cheap and easily available
(c) so that pulse may move through it easily
(d) so that time taken by pulse to move from one end of string to other may be accurately determined

Question 14:
A pulse was created in a slinky/string of length 4 m by a group of four students. They observed that it returned after reflection at the point of creation 6 times in 10 seconds and calculated the
speed as follows:

The correct speed was calculated by the student
(a) A
(b) B
(c) C
(d) D

Question 15:
A strong transverse horizontal pulse, created at one end of a string, is observed to complete five journeys along its length before fading out. The initial and final readings, on a stop-clock used in the experiment are as shown here. If the length of the string is L metre, the speed of the pulse through the string, is

Question 16:
A student fixes two ends of a rope to two rigid supports A and B placed at a distance of 7.5 m. He then gives a transverse horizontal jerk to create a pulse in the rope which moves from A
to B, B to A, again from A to B and B to A…….and so on. The moment he gives jerk to the rope, his friend immediately starts the stop watch. The pulse dies after completing three to and fro journeys from A to B and back. The pulse takes 1 minute and 15 seconds to complete its entire journey. The speed of the pulse moving on the rope is

(a) 0.6 ms-1
(b) 0.3 ms-1
(c) 0.2 ms-1
(d) 0.1 ms-1

Question 17:
While doing the experiment on measuring the velocity of a pulse through a stretched string, a student had to choose between using a

1. thick silk string and a thick cotton string
2. stop clock and a table clock

The combination of choice that he should prefer is
(a) silk string and stop clock
(b) silk string and table clock
(c) cotton string and stop clock
(d) cotton string and table clock

Multiple Choice Questions/VSA

1. (d)
2. (b)
3. (a)
4. (d)
5. (c)
6. (b)
7. (d)
8. (b)
9. (b)
10. (c)
11. (a)
12. (a)
13. (d)
14. (b)
15. (b)
16. (a)
17. (c)