$(a)$ $A$ boy strikes one end of a long pipe with a stone. Another boy who keeps his ear close to the other end of the pipe hears two sounds in a short interval of time. Explain why.
$(b)$ List two uses of ultrasonics.

(N/A) When a boy strikes an iron pipe,sound is produced. The sound waves travel through both the air and the material of the pipe. Since the speed of sound in iron is significantly greater than the speed of sound in air,the sound travels faster through the iron pipe than through the air. Therefore,the boy at the other end hears two distinct sounds: one that traveled through the iron pipe and arrived first,and another that traveled through the air and arrived later.
$(b)$ Two uses of ultrasonic waves are:
$(i)$ They are used to determine the depth of the sea using $SONAR$ (Sound Navigation and Ranging).
$(ii)$ They are used in medical diagnostics,such as ultrasonography,to detect diseases or abnormalities in the human body.