No, the force applied by both boys is equal. So, net force will be zero and hence friction force will not come into play.

Ultrasound refers to sound that has a frequency above the audible range (20,000 Hz) of the human ear.

Mass of an object remains constant at all the places, so mass of astronaut will not change. Weight of an object depends upon the gravity, at the moon gravity is lesser than earth, so weight of astronaut will decrease.

SI unit of pressure is N / m² or pascal.

Temporary deafness is an ill effect of noise pollution.

Through water as a liquid medium, both whales and dolphins hear the sound.

Across:
1. Oscillation
2. Hertz
3. Eardrum
4. Music
Down:
5. Pitch
6. Audible
7. Noise
8. Ultrasonic

As we know that the speed of sound in air is about 340 m/s and the speed of light in air is 300,000,000 m/s. It means that sound travels at a slow speed while light travels much faster than sound.

So, though sound may take a few seconds to travel a distance of a few hundred metres, light will take practically negligible time to reach a distance of even a few kilometres. e.g. in the rainy season, the flash of lightning is observed first and the sound of thunder is heard a little later (though both are produced at the same time in clouds) (see figure). It is because of the very high speed of light that we see the flash of lightning first and it is due to comparatively low speed of sound that the thunder is heard a little later.

Friction is called a necessary evil because we cannot do anything without friction and it also shows effects. No movement would come to an end without friction. No body would rest without friction. We cannot write with pen or pencil if there is no friction. If an object starts moving, it would never stop if there was no friction. It is also necessary for grip and bringing movements to rest.

(i) Vibration of metal surface
(ii) Flapping of wings
(iii) Air vibrates to produce sound.
(iv) Vibrations of membrane
(v) Vibrations of strings
(vi) Stretched membrane
(vii) Air column

Effect of the force in given situations:
(i) While taking a penalty kick in football the player applies a force on the ball. Before being hit, the ball was at rest and so its speed was zero. It indicates that an object can move from rest with effect of force.
(ii) A car is moving on the road at a slow speed. Applying force on the accelerator will increase the speed of the car.
(iii)Take lump of dough in a plate. Make chapatis from that dough by rolling roller and putting pressure on it. It changes the shape by force.

The force responsible for wearing out tyres of motor vehicle is force of friction.