Mach number basics & the sound barrier

5:00want to travel faster. Yes. Right. This is something that'll give us access to faster air travel. It would be really nice if we just, you know, got to go to, if I could go to India in an hour and visit my grandmother, that would be really awesome, right? Pretty chill. >> But the age of the Concord is gone and now it's sort of coming back. Supersonic travel is sort of coming back. Um, hypersonic travel is something that is on the docket, >> but it's a little more non-trivial compared to supersonic. Okay, so in order to get into that, there's some fundamental physics involved that I think is very, very cool. Let's talk about the mock number. You know about the mock number, right? It's the ratio of the speed of your object divided by

5:40the speed of sound. The speed of sound is about 750 mph. Planes usually operate at around like what 200 300ish. Yeah. >> Right. Um and at the altitude that planes are at, you're a little bit lower. The speed of sound is around 600 650, right? Because the density is lower. But at the end of the day, the the air behaves differently when you are traveling at close to or above the speed of sound. Okay? And one of my favorite movies of all time is The Right Stuff. It was a movie about the early US Army tests out in um Edwards Air Force Base. And there's a great scene where Chuck Joerger is trying to conquer the demon

6:23in the sky. And the demon in the sky is the sound barrier. Okay? Because back then no one had gone faster than the speed of sound and it seemed non-trivial whether you could. Now engineers were like of course you can because a bullet goes faster than the speed of sound and uh you know Erns Mock had >> that famous photograph of the bullet going through the speed of sound that we had covered in some earlier episodes. So >> it's it's obvious that stuff can go faster than the speed of sound and the atmosphere permits it. Okay. It's less obvious whether something as big as a airplane that's carrying a human being can go that fast. >> It there's a scaling issue, a potential scaling issue.

7:03>> There's a potential scaling issue because what ends up happening is when you go faster than the speed towards the speed of sound, the disturbance, the pressure waves that the the disturbances that you're creating with your object are going to move away from your object at the speed of sound. That's what sound is. Sound is the speed at which some disturbance moves through air. >> But if you're piercing through that barrier, right? Then the disturbance is sort of catching up with you. You're or you're catching up with that disturbance, right? And you're piling up air in front of you. >> And air goes from something called incompressible an incompressible fluid,

7:45which is >> a way a way of saying that basically air doesn't change its density as we change its pressure and temperature to something where it does. Now you can squish air. Okay? And what ends up happening is at the speed of sound, you get these things called the mock cone where the wavefront of all of that disturbance becomes a nice little conical wavefront >> in your >> device like right around the device that's going through. >> Right. This picture here is of >> a model airplane in a wind tunnel. Yes. Where the wind is moving faster than the speed of sound. Right. So then relative the the the object is moving faster than the speed of sound and you're getting

8:25that really nice mock cone. Right. Right. And the angle of that cone tells you how fast you're going.