CBSE Class 7 Science Advanced Physics Sound Very Short Answer Questions Set 01

Very Short Answer Questions

Question 1. Fill in the blank.
The to and fro motion of an object is called ________.
Answer:
vibration (oscillation)
In simple words: Moving back and forth about a fixed position is a vibration.

Teacher's Note:
1. Another name for it is oscillatory or vibratory motion.
2. The rubber band and the tuning fork prongs show this motion.

Question 2. Fill in the blank.
s⁻¹ is the unit of ________.
Answer:
frequency. One s⁻¹ is the same as 1 hertz.
In simple words: Per second means the number of times something happens each second, which is frequency.

Teacher's Note:
1. Frequency = number of oscillations ÷ time, so its unit is per second.
2. The unit has a special name: hertz.

Question 3. Fill in the blank.
The time taken to perform 45 vibrations is ________ times the time taken for a single vibration.
Answer:
45. Every vibration takes the same time T, so 45 vibrations take 45 T.
In simple words: Forty-five equal vibrations need forty-five times as long as one.

Teacher's Note:
1. The time period is constant for a given vibrating body.
2. Total time = number of vibrations × time period.

Question 4. Fill in the blank.
If a body vibrates 50 times in a second, then its frequency is ________.
Answer:
50 Hz. Frequency is the number of vibrations per second.
In simple words: Fifty vibrations in one second means a frequency of fifty hertz.

Teacher's Note:
1. The printed key says 500 Hz, which is a misprint; the correct value is 50 Hz.
2. Vibrations in one second are directly the frequency.

Question 5. Fill in the blank.
Sitar, veena and violin are some ________ instruments.
Answer:
stringed. Their sound comes from vibrating strings.
In simple words: These instruments have strings that are plucked or bowed.

Teacher's Note:
1. Changing the length, tension or thickness of the string changes the pitch.
2. Wind and percussion instruments work differently.

Question 6. Fill in the blank.
The sounds of frequencies greater than ________ are called ultrasonic sounds.
Answer:
20 kHz (20 000 Hz)
In simple words: Above 20 000 Hz humans cannot hear, and the sound is called ultrasonic.

Teacher's Note:
1. Infrasonic sounds are below 20 Hz.
2. Bats and dogs can respond to ultrasonic sounds.

Question 7. Fill in the blank.
Sound travels much ________ than light in a given medium.
Answer:
slower. Sound travels at about 330 m s⁻¹ in air while light travels at 3 × 10⁸ m s⁻¹.
In simple words: Light is enormously faster than sound.

Teacher's Note:
1. This is why we see lightning before hearing thunder.
2. The difference is about a million times.

Question 8. Fill in the blank.
In general, the voice of a woman has a ________ than that of a man.
Answer:
higher frequency (higher pitch)
In simple words: A woman's voice is shriller because its frequency is higher.

Teacher's Note:
1. Pitch depends on frequency.
2. Thinner and tighter vocal cords give a higher frequency.

Question 9. Fill in the blank.
The velocity of sound is ________ in air compared to that of water.
Answer:
less. It is about 330 m s⁻¹ in air and about 1500 m s⁻¹ in water.
In simple words: Sound is slower in air than in water.

Teacher's Note:
1. Sound speed order: solids, then liquids, then gases.
2. Denser, more tightly packed media pass the vibrations on quicker.

Question 10. ________ are used to detect flaws inside a metal.
Answer:
Ultrasonic sounds (ultrasound waves)
In simple words: Ultrasound reflects from cracks inside metal, which shows where they are.

Teacher's Note:
1. The reflected ultrasound changes where there is a hole or crack.
2. This avoids cutting the metal open.

Teacher's Note:
1. Start with the pairs you are sure of.
2. Cross out the options as you use them.

Teacher's Note:
1. Mega means a million.
2. Loudness always goes with the decibel.

Teacher's Note:
1. Units help in matching: amplitude is a length, so m.
2. f = 1/T is the link for frequency.

Question 14. How is sound produced?
Answer:
Sound is produced by vibrating bodies. For example, a plucked rubber band or a struck tuning fork moves to and fro rapidly and gives out sound.
In simple words: Anything that vibrates quickly enough makes sound.

Teacher's Note:
1. The pith ball test shows the prongs of a sounding fork are vibrating.
2. If the vibration stops, the sound stops.

Question 15. What is the time period of a simple pendulum?
Answer:
The time taken by the bob to complete one oscillation (one full to-and-fro swing). It is denoted by T and measured in seconds.
In simple words: It is the time for one complete swing.

Teacher's Note:
1. Measure the time for 20 swings and divide by 20 for accuracy.
2. T = 1/f.

Question 16. What is the length of a simple pendulum?
Answer:
It is the distance from the point of suspension to the centre of the bob.
In simple words: Measure from where the thread is tied to the middle of the ball.

Teacher's Note:
1. The length decides the time period of the pendulum.
2. Measure to the centre, not the bottom of the bob.

Question 17. What is frequency of oscillation?
Answer:
The number of oscillations made in one second, f = number of oscillations ÷ time taken = 1/T. It is measured in hertz (Hz).
In simple words: It tells how many swings take place each second.

Teacher's Note:
1. A higher frequency means faster vibrations.
2. Frequency and time period are reciprocals.

Question 18. What is meant by one oscillation of a simple pendulum?
Answer:
One oscillation is the motion of the bob from one extreme position to the other extreme and back to the first extreme, that is, one complete to-and-fro swing.
In simple words: One full swing out and back is one oscillation.

Teacher's Note:
1. Going only one way is half an oscillation.
2. Count a swing only when the bob returns to where it started.

Question 19. What is the SI unit of frequency?
Answer:
The hertz (Hz), equal to one oscillation per second (s⁻¹).
In simple words: Hertz means vibrations per second.

Teacher's Note:
1. Larger units are kHz and MHz.
2. 1 kHz = 1000 Hz.