Breaking the Sound Barrier

[peter]

We all know that when you flick a whip, the wave of movement travels down the length getting smaller and faster as it goes until 'Crack!' - it breaks the sound barrier, or to put it another way it's speed passes that at which sound waves travel through air. OK - fair enough, but what does that mean in terms of that crack? Is it that the air molecules are pushed toward your ear faster than at the point which the ones behind can catch up - and when they do the resulting collision produces the tell-tale crack? What's going on here?

[Apologies if I've asked this question before - my memory sucks these days! ]

[Hashi Lebwohl]

If you slowed the tip of the whip down and magnified both it and what is going on around it, the tip begins to move so quickly that the air it pushes out of the way creates a pressure wave, just like what happens with an explosion. The pressure in front of the wave builds as the quickly-moving air molecules are pushed into the slowly-moving molecules; the wave itself is what you hear as the "crack". The pressure behind the wave is significantly reduced since so many air molecules have been pushed out of the way.

Enter the phrase "slow motion explosion" in the search bar for YouTube and you can see lots of examples of shockwaves.

[wayfriend]

It's a sonic boom.

[peter]

Is that the same as the bursting of a balloon then? Got to be I guess....

Seem to remember that when Concord flew overhead (ahh, those were the days! ) you had a series of such booms in quick succession - due I guess to the repetition of the process Hashi describes above. Wonder if you analyze sonic boom structure whether they all have a tell-tale signature form? Will head to YouTube to see the video (love those sciencey vids!)