Sound and waves exam q's

Question 1

In the investigation of the variation of the fundamental frequency, explain why the tension in the string was kept constant during the investigation?

Answer 1

• The natural frequency of a string is also related to the tension in te string. To investigate the relationship between natural frequency and length you must keep the tension constant.
• When you investigate the relationship between two variables keep all other factors constant.

Question 2

In the investigation of the variation of the fundamental frequency, how did the student know that the string was vibrating at its fundamental frequency?

Answer 2

The paper rider falls off the centre of the string due to the antinode at the centre of the string and the nodes at the bridge. (Draw diagram)

Question 3

Explain the term constructive interference.

Answer 3

When two waves combine to give a wave of greater amplitude.

Question 4

Explain the term coherent.

Answer 4

Sources which emit waves with the same frequency and the waves are in plane.

Question 5

What is the condition necessary for destructive interference to take place when waves from two coherent sources meet?

Answer 5

When the crest of one wave combines with the trough of another wave, i.e. when the waves meet and combine they have a difference phase of λ/2.

Question 6

What is the Doppler effect?

Answer 6

The apparent change in the frequency of a wave due to the relative motion between a source or an observer.

Question 7

Give two applications for the Doppler effect.

Answer 7

• To measure the speed of a car

- To measure the speed of stars (red shift)

Question 8

When investigating the variation of the fundamental frequency f of a stretched string with its tension T, how was the tension measured?

Answer 8

By calculating the weight of the pan and its contents.

Question 9

When investigating the variation of the fundamental frequency f of a stretched string with its tension T, what relationship did the student discover?

Answer 9

Frequency is proportional to the square root of tension. f ∝ 1/l

Question 10

When investigating the variation of the fundamental frequency f of a stretched string with its tension T, why was is necessary to keep the length constant?

Answer 10

Since frequency also depends on length you must keep length constant when investigating the relationship between frequency and tension.

Question 11

When investigating the variation of the fundamental frequency f of a stretched string with its tension T, how did the student know that the string was vibrating at its fundamental frequency.

Answer 11

The paper rider falls off the centre of the string due to the antinode at the centre of the string and the nodes at the bridge. (Diagram needed)

Question 12

State the relationship between frequency of the string and its length and explain how the graph verifies it (straight through origin).

Answer 12

• Frequency is inversely proportional to the length. (f ∝ 1/l)
• Since the graph was a straight line through the origin then f ∝ 1/l

Question 13

Give two factors other than length that affect the frequency of a stretched string.

Answer 13

Tension and mass per unit length.

Question 14

What is the frequency of a second harmonic?

Answer 14

Double that of the first.

Question 15

Define sound intensity.

Answer 15

The sound energy per second per unit area.

Question 16

The human ear is more sensitive to certain frequencies of sound. How is this taken into account when measuring sound intensity levels.

Answer 16

Use is made of a decibel adapted scale (dBA scale).

Question 17

What does the quality of a musical note depend?

Answer 17

Depends on the number of overtones (harmonics) present in a note.

Question 18

What does the loudness of a musical note depend?

Answer 18

Depends on the amplitude of a sound wave.

Question 19

What is the Doppler effect?

Answer 19

The apparent change in the frequency of a wave due to the relative motion between a source or an observer.

Question 20

Give two applications of the Doppler effect.

Answer 20

• To estimate the speed of stars

- To calculate the speed of a moving car

Question 21

What is the Doppler effect?

Answer 21

The apparent change in the frequency of a wave due to the relative motion between a source or an observer.

Question 22

What causes the red shift in the spectrum of a distant star?

Answer 22

• The star was moving relatively close to earth.
• A longer wavelength is observed if the star moves away from earth.
• A shorter wavelength is observed is the star moves towards the earth.

Question 23

Give an application of the Doppler effect.

Answer 23

Blood flow measurement

Question 24

What is meant by the term resonance?

Answer 24

The transfer of energy between two systems of similar natural frequencies.

Question 25

What is the Doppler effect?

Answer 25

The apparent change in the frequency of a wave due to the relative motion between a source or an observer.

Question 26

State two applications of the Doppler effect.

Answer 26

• Police use speed cameras to measure speeds of cars

- Astronomers use Doppler effect to measure the speeds of stars

Question 27

What are stationary waves?

Answer 27

Wave where there is no net transfer of energy.

Question 28

How are stationary waves produced?

Answer 28

Produced when two progressive waves of the same amplitude, same frequency, same speed and moving in opposite direction meet.

Question 29

What is resonance?

Answer 29

The transfer of energy between two systems of similar natural frequencies.

Question 30

The frequency of a stretched string in a guitar depends on the tension of the string and on two other factors. What are the two other factors?

Answer 30

• Length of the string

- The mass per unit length of the string

Question 31

What is the Doppler effect?

Answer 31

The apparent change in the frequency of a wave due to the relative motion between a source or an observer.

Question 32

Define centripetal force.

Answer 32

The force acting towards the centre of the circle for an object undergoing circular motion.

Question 33

What is resonance?

Answer 33

The transfer of energy between two systems of similar/the same natural frequencies.

Question 34

Define sound intensity.

Answer 34

The sound energy per second per unit area.

Question 35

In the investigation of the variation of the fundamental frequency f of a stretched string with its tension T, why was the length of the string kept constant?

Answer 35

• The natural frequency of a string is also related to the tension in te string. To investigate the relationship between natural frequency and length you must keep the tension constant.
• When you investigate the relationship between two variables keep all other factors constant.

Question 36

In an experiment to measure the speed of sound in air, describe how was the length of the column of air was adjusted.

Answer 36

The open pipe can be raised or lowered in the water to change the length of the column of air.

Question 37

In an experiment to measure the speed of sound in air, describe how was the length of the column of air was adjusted.

Answer 37

The open pipe can be raised or lowered in the water to change the length of the column of air. (Diagram)

Question 38

In an experiment to measure the speed of sound in air, describe how the frequency of the column of air was measured.

Answer 38

When the column of air is vibrating at its first harmonic the frequency of the column of air equals the frequency of the tuning fork. Read the frequency on the tuning fork.