Question

A person blows into the open-end of a long pipe. As a result, a high pressure pulse of air travels down the pipe. When this pulse reaches the other end of the pipe, what will be the possible reaction (s) from the other end?
A.A high pressure pulse starts traveling up the pipe, if the other end of the pipe is open.
B.A low pressure pulse starts traveling up the pipe, if the other end of the pipe is open.
C.A low pressure pulse starts traveling up the pipe, if the other end of the pipe is closed.
D.A high pressure pulse starts traveling up the pipe, if the other end of the pipe is closed.

Answer 1

The pressure developed at the ends of a pipe depends on the type of waves formed inside them. The wave type is determined by the pipe whether open or closed at the other end. So, this situation will have two conditions to be dealt with.

Complete answer:
Let us consider a pipe of some length ‘L’. One end of the pipe is open, from which the disturbance is produced and the other end is considered to be open in case I and closed in case II.
Depending on the length of the pipe there can be ‘n’ number of modes of vibration of air particles in the pipe. The nature of vibration depends on the condition whether the other end is open or closed. So, let us consider both the cases one-by-one.
Case I (The other end is open): An open end in the opposite side of the source of vibration or disturbance can create an antinode at this end. This situation is illustrated below –

The above figure shows a third harmonic vibration of the wave found with high pressure at one open end moving to the other end. The low pressure is created by the disturbance creating an opposite condition as that of the first end.
Case II (The other end is closed): Let us consider a system in which the wave travels as shown in the below figure. It starts with a low pressure undergoes fundamental vibrations and ends at the closed end and returns in the same manner. i.e., the wave created is maintained in the pipe as it is. The condition which we have here has a high pressure open end, that means the other end also creates a high pressure region up in the pipe.

Therefore, we can conclude that for a high pressure disturbance at open end of a pipe results in -
a).A low-pressure in the pipe at the other end, if the pipe is open at both ends.
b).A high-pressure in the pipe at the other end, if the pipe is closed at this end.

The correct answers are given by option B and C.

Additional Information:
The wave propagation in string is a transverse wave, which is often mistreated as longitudinal.

Note:
We must not get confused with the wave propagation of a transverse wave in this situation. The pressure conditions related to them are different from the longitudinal waves like sound. The fundamental modes of vibration are different for these waves.