Class 9 Science Sound: NCERT Solutions for Question 7

Explain the working and application of a sonar.

Explain how sound is produced by your school bell.

Explain how defects in a metal block can be detected using ultrasound.

A sound wave travels at a speed of 339 m s^-1. If its wavelength is 1.5 cm, what is the frequency of the wave? Will it be audible?

Distinguish between loudness and intensity of sound.

A sonar device on a submarine sends out a signal and receives an echo 5 s later. Calculate the speed of sound in water if the distance of the object from the submarine is 3625 m.

Which characteristic of the sound helps you to identify your friend by his voice while sitting with others in a dark room?

How is ultrasound used for cleaning?

Which wave property determines
(a) loudness,    (b) pitch?

Explain how the human ear works.

Which of the following has more inertia: (a) a rubber ball and a stone of the same size? (b) a bicycle and a train? (c) a five-rupees coin and a one-rupee coin?

State the universal law of gravitation.

Which of the following are matter?

Chair, air, love, smell, hate, almonds, thought, cold, cold-drink, smell of perfume.

A force of 7 N acts on an object. The displacement is, say 8 m, in the direction of the force (Fig. 11.3). Let us take it that the force acts on the object through the displacement. What is the work done in this case?

What is meant by a pure substance?

In a reaction, 5.3 g of sodium carbonate reacted with 6 g of ethanoic acid. The products were 2.2 g of carbon dioxide, 0.9 g water and 8.2 g of sodium observations are in agreement with the law of conservation of mass.

sodium carbonate + ethanoic acid → sodium ethanoate + carbon dioxide + water

What are canal rays?

State any two conditions essential for good health.

How is our atmosphere different from the atmospheres on Venus and Mars?

Who discovered cells, and how?

A trolley, while going down an inclined plane, has an acceleration of 2 cm s^-2. What will be its velocity 3 s after the start?

What type of clothes should we wear in summer?

On the basis of Thomson’s model of an atom, explain how the atom is neutral as a whole.

Write down the formulae of
(i) sodium oxide
(ii) aluminium chloride
(iii) sodium suphide
(iv) magnesium hydroxide

What are polyatomic ions? Give examples.

Certain force acting on a 20 kg mass changes its velocity from 5 m s^-1 to 2 m s^-1. Calculate the work done by the force.

On the basis of Rutherford’s model of an atom, which sub-atomic particle is present in the nucleus of an atom?

An object experiences a net zero external unbalanced force. Is it possible for the object to be travelling with a non-zero velocity? If yes, state the conditions that must be placed on the magnitude and direction of the velocity. If no, provide a reason.

When will you say a body is in

(i) uniform acceleration? (ii) non-uniform acceleration?

Explain any one method of crop production which ensures high yield.