Question of Exercise 2

Gravitation of Class 9

A particle is taken to a height R above the earth’s surface, where R is the radius of the earth. The acceleration due to gravity there is :


Option 1 2.45 m/s2

Option 2 4.9 m/s2

Option 3 9.8 m/s2

Option 4 19.6 m/s2

Frequently Asked Questions

Define force Write its SI unit



  • A physical factor that alters or tends to modify an object's state of rest or motion is called force.
  • Newton is the SI unit of force.
  • It is denoted by ‘N’.

Final answer:

Force is a factor that changes the state of the body from rest or motion and the SI unit of force is Newton (N).


What are ultrasonic waves

A: Sound waves having frequency shorter than audio frequency range

B: Sound waves produced in a vaccum

C: Sound waves having frequency higher than audio - frequency range

D: None of these




The audible frequency range for the human ear is 20Hz to 20000Hz; sound waves over 20000Hz are ultrasonic, while sound waves below 20Hz are infrasonic. 

Ultrasonic waves are defined as sound waves with a frequency greater than the audio frequency range (20.000Hz).

Final answer:

Hence, option C is correct, i.e., Sound waves having frequency higher than audio - frequency range.


Why is sound wave called a longitudinal wave



Sound wave is called a longitudinal wave because sound waves propagate in a medium as a series of compressions and rarefractions and particles of medium move in a direction parallel to the direction of propagation of the wave.

Final Answer:

In sound wave, the vibrations of the particles of  the  medium are along the direction of wave propagation, so sound waves in air are longitudinal.Therefore, sound wave is called a longitudinal wave.



Define the following terms Time period



Time period is denoted by T.

It can be defined as the time taken by one particle to complete one vibration.

Also, the time taken by any vibratory object to complete one oscillation is called time period.

Its SI unit is second.

Final Answer:

Time period is the time taken by any vibratory object to complete one oscillation.


The number of vibration/ oscillation executed per second is known as

A: Frequency

B: Amplitude

C: Wave length

D: Pitch



  • Frequency is the number of times a repeating occurrence occurs per unit of time. 
  • To distinguish it from spatial frequency, it's sometimes called temporal frequency or ordinary frequency. 
  • The Hertz (Hz), unit of frequency equals one occurrence every second. 
  • Because it represents the length of one cycle in a recurring occurrence, the period is the reciprocal of the frequency.

Final answer:

Hence, option A is correct, i.e., Frequency.

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