Name two musical instruments which produce sound by vibrating strings?

Boojho saw a cracker burst at night at a distance from his house. He heard the sound of the cracker a little later after seeing the cracker burst. Give reason for the delay in hearing the sound.

When we hear a sound, does any part of our body vibrate? Name the part.

A simple pendulum makes 10 oscillations in 20 seconds. What is the time period and frequency of its oscillation?

We have learnt that vibration is necessary for producing sound. Explain why the sound produced by every vibrating body cannot be heard by us?