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We now derive Eq. 17-3 by direct application of Newton’s laws. Let a single pulse in which air is compressed travel (from right to left) with speed v through the air in a long tube,like that in Fig.16-2.Let us run along with the pulse at that speed, so that the pulse appears to stand still in our reference frame. Figure 17-3 a shows the situation as it is viewed from that frame.The pulse is standing still,and air is moving at speed v through it from left to right.

In that part that says that the pulse is going from right to left with speed v, how does the air move from left to right at the same speed v ? is suppoused to be the opposite ?

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The text says the pulse moves from right to left and you move with it at the same speed.

That means the air, which stands still in the rest frame, moves, in your reference frame, from left to right. Or, in other words, the not-moving air seems to move from left to right for you, because you are moving from right to left.

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