Solution to Activity 7.7 of NCERT Class 9 Science Chapter 7 Motion

7. Complete Activity 7.7 (Page 76).

• At a time when it is cloudy, there may be frequent thunder and lightning. The sound of thunder takes some time to reach you after you see the lightning.

• Can you answer why this happens?

• Measure this time interval using a digital wrist watch or a stop watch.

• Calculate the distance of the nearest point of lightning. (Speed of sound in air = 346 m s^-1.)

Answer:

The sound of thunder takes some time to reach you after you see the lightning because sound travels much slower than light. When lightning occurs, light travels almost instantaneously to your eyes. However, sound travels much slower, at about 343 meters per second, so it takes some time for the sound of thunder to reach your ears.

The time interval can be measured as 5 seconds using a stop watch.

Sound takes 5 seconds to reach and speed of sound in air is 346 m s^-1.

Therefore, distance travelled by sound = 346 m s^-1 × 5 s = 1730 m = 1.73 km. Therefore, distance of the nearest point of lightning is 1.73 km.

“• At a time when it is cloudy, there may be frequent thunder and lightning. The sound of thunder takes some time to reach you after you see the lightning.

• Can you answer why this happens?

• Measure this time interval using a digital wrist watch or a stop watch.

• Calculate the distance of the nearest point of lightning. (Speed of sound in air = 346 m s^-1.)” – Solved.