NCERT 8th Class (CBSE) Science: Sound

Question: Describe an experiment to show that sound can travel though liquid?

Answer: Experiment to show that sound can travel through liquids:

1. Let us take a tub filled with water. Hold a bell in one hand and dip it in water.
2. Keep one of your ears (caution: water should not enter your ear) gently on the surface of water and ring the bell inside the water.
3. We will be able to hear the sound clearly. This shows that sound can travel through liquids also.

Question: Describe an experiment to show that sound cannot travel though vacuum?

Answer: Experiment to show that sound cannot travel through vacuum:

1. Let us take a container with a tight closing lid.
2. Make a hole at the bottom of the container.
3. Connect a vacuum pump to this hole with a rubber tube.
4. The vacuum pump is used to extract air from the container.
5. Put a cell phone inside the container and close it with the lid.
6. Call the number of the cell phone so that it rings.
7. We can hear the sound clearly.
8. Now, start the vacuum pump and extract the air from the container.
9. Call the number of the cell phone again.
10. We cannot hear the sound now. This experiment proves that sound cannot travel through vacuum.

Question: Can sound travel through solid medium. Prove.

Answer: Following experiment shows that sound travel through solid medium:

1. Keep one of your ears against one end of your desk.
2. Close the other ear firmly with the palm of your hand.
3. Tell a friend to tap the desk at the other end with his hand.
4. You will be able to hear the sound loud and clear.
5. The sound has travelled to your ear through the desk. This shows that sound can travel through solids also.

Question: An explosion occurs on the moon. Will it be (a) seen (b) heard on the earth instantly or after some time?

Answer: If an explosion occurs on the moon, it will be seen after approximately 1.2 second, but not heard because there is no air on moon.

Question: In the game hide and seek, we can be blind-folded person guess which player is closest to him?

Answer: In the game hide and seek, the blind-folded person can guess which player is closest to him by hearing sound created by the player.