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00:00This is me on top of a speeding train, and this is me in a hot air balloon to answer
00:04some of the most debated science questions on the internet.
00:07For example, my hot air balloon pilot here says he can land me anywhere I want, and yet there's nothing
00:11here that resembles a steering wheel.
00:13Which begs the question, how do you actually steer these things?
00:16So we steer...
00:17And now I'm on top of the train to learn once and for all, why do you land in the
00:21same spot if you jump inside the train, but not if you jump on top of the train?
00:25But I'm not stopping there.
00:28Because today we're gonna investigate five more physics and engineering puzzles using simple demonstrations as we go.
00:34Because our goal by the end of this video is not for you just to know the right answers, but
00:38more importantly for you to understand why they're the right answers.
00:41To kick things off, I've got a science riddle for you that most people get wrong.
00:45What is the first man-made object to break the sound barrier?
00:49In other words, an object that travels more than 767 miles per hour through the air,
00:53exceeding the speed of sound, thereby creating a super loud sonic boom.
00:57So what do you think?
00:58Most people I ask say either a bullet, or a rocket, or even a jet plane.
01:03But this set goes way back.
01:04Like 5,000 years way back.
01:06Because the first man-made object to break the speed of sound...
01:11is a whip.
01:13The sound you just heard was literally a sonic boom.
01:18Which means at the peak of its motion, the tip of the whip is moving faster than 767 miles per
01:27hour.
01:27Okay, so if you got that right, time for a bonus question.
01:30What's almost surely the first time you yourself heard a sonic boom?
01:34Here's your clue.
01:35It's a near statistical certainty that everyone has the same answer for this going back millions of years.
01:40That's right, thunder.
01:42Basically, the lightning super heats the air five times hotter than the surface of the sun.
01:46And this causes the air to expand so rapidly, it breaks the sound barrier once again, creating a sonic boom.
01:52And if that juicy nugget of knowledge just made your brain feel really good...
01:57Ow!
01:58Make that sound cooler in post.
02:01Yeah.
02:02Buckle up, because we've got six more to go, including...
02:05At number two, have you ever noticed when grabbing drinks from a cooler,
02:09that even though they have the same amount of liquid inside,
02:11regular coke sinks, yet diet coke floats?
02:15Now, why would that be?
02:17Well, our first clue is that we know less dense objects like this cork float,
02:21and more dense objects like this bolt sink.
02:24However, that's not only true about different objects, it's true about different liquids, too.
02:28Like for salad dressing, how the less dense oil floats on the more dense vinegar.
02:33And sure enough, when we compare the respective labels, you'll notice one glaring difference.
02:37Regular coke has more than three tablespoons of real sugar,
02:40which makes this can more dense than the diet coke, which just has artificial sweetener.
02:44That means this one's just a little less dense than water, causing it to float,
02:48while this one's just a little more dense than water, causing it to sink.
02:51But we can take this to the next level.
02:53Here I have a wall that I filled with eight different household liquids,
02:56from honey, mmm, dish soap, to baby oil.
02:59And they naturally stack themselves with increasing density as you go down.
03:03So if we dump, let's say, a bunch of golf balls in,
03:05they'll sink like a coke can through all the layers,
03:08until they hit a layer that's more dense than they are,
03:10at which point the golf balls stop falling and come to rest on top of that layer.
03:14Are you kidding me?
03:16Which means you can have a different item come to rest on every layer.
03:19Whoa!
03:21As long as you spend way more time than you care to admit.
03:23Cool!
03:24Testing way more items than you care to admit.
03:27This one's a dice roll.
03:28But in the end, it's not hard to rationalize the wasted time when it looks this cool.
03:32By the way, you can make a simple version of this yourself at home,
03:35by filling a drinking glass with baby oil and colored water,
03:38and then plopping in a Lego guy to ride the ocean wave.
03:40So this obviously works with household liquids.
03:43But what if the liquid was metal?
03:45Like this.
03:45Because this bowl is 24 pounds of mercury.
03:51It's hard to even submerge my hand in here.
03:53Which means, if I take an actual cast iron anvil,
03:57it should float like a Diet Coke.
03:59Let's test it out.
04:02Look at that.
04:03I mean, I understand the science behind this,
04:06but it still breaks your brain.
04:08This is actual cast iron.
04:10It's heavy.
04:11And yet when you put it in mercury, it bobs like a cork.
04:14Let's go bobbing for anvils.
04:16For number 3, a while back I read about a famous mathematician for the 1600s,
04:20who noticed that when he would hang up multiple pendulum clocks in his house,
04:23they'd eventually sync up their swings,
04:25and then they'd just stay that way.
04:27So he wondered, just like I did when I first read this,
04:29how are these purely mechanical clocks that weren't touching,
04:32somehow communicating with each other and after a period of time,
04:35agreeing to swing in perfect unison?
04:37And since the unofficial motto of Crunch Labs is
04:40think like an engineer,
04:41I decided to create an experiment to test this for myself.
04:44So we got four clocks and put them on a wall.
04:46And after four days,
04:50nothing happened.
04:51But when you're thinking like an engineer,
04:52you know that's not a failure, that's a clue.
04:55So I re-read the story and picked up on an important detail I'd missed the first time.
04:59The clocks weren't mounted on a wall in the way we think of today.
05:02It said they were mounted on the same wooden beam that made up part of the wall.
05:05And given the construction methods of the 1600s,
05:08there's a good chance that board had a little wiggle to it.
05:10And that wiggle is a very important detail.
05:13Here's what I mean.
05:14If I swing this sledgehammer pendulum below me while standing on firm ground,
05:18then it's just the sledgehammer pendulum that you see moving.
05:22But if I stand on a skateboard where I don't have firm footing,
05:24now you can see the pendulum is in fact pushing back and moving me too.
05:28Now here's the wild part.
05:30Because the clock is securely attached to the beam,
05:32that beam acts just a tiny bit like the skateboard.
05:35So those swinging pendulums are ever so slightly actually moving the wall.
05:40So that means as the clocks are just slightly pushing back and nudging the beam,
05:43they start to influence and nudge each other too.
05:46So if there's a clock that's greatly out of sync with the others,
05:49over time those little nudges get it to line up.
05:51So after discovering all this, we took those four clocks,
05:54and this time attached them all to a beam,
05:56which was attached to the wall with a little bit of wiggle built in.
05:59And this time after four days, nothing happened.
06:03Which once again was a clue, not a failure.
06:06Because it turns out for this to actually work with pendulum clocks,
06:09you need a very precise ratio of clock to pendulum mass like this set up here.
06:14So while I couldn't get my hands on the perfect pendulum clock,
06:16here's another way to observe the same phenomenon with 140 pendulums
06:20all sitting on a board, I can freely wiggle back and forth.
06:23All right, let's give it a shot.
06:25A little help here.
06:32This is truly one of the coolest things I've ever seen.
06:37It's like a bunch of storm troopers.
06:41And just to give you a sense of how quickly they sync up,
06:44here's an uninterrupted take so you can see for yourself
06:46as the chaos turns to order in less than a minute.
06:57So as you can see, even though we started them totally random in less than a minute,
07:01those nudges do in fact influence each other until they're all swinging together,
07:06just like the clocks 400 years ago in the mathematician's house.
07:11Coming in at number 4,
07:12Have you ever noticed every time you see a full moon,
07:14it always looks the same?
07:16In other words, it's the same side, even in pictures.
07:19Well, what about the other side?
07:20Well, it turns out we don't see the other side
07:23because the moon is rotating perfectly in sync with its orbit around the Earth.
07:27So we only ever see the googly eyes.
07:30But that's wild!
07:31Like what are the chances that those would sync up perfectly?
07:34Well, we call this tidal locking, and while it seems unlikely,
07:37you see this with pretty much all the other planets and their moons in our solar system too.
07:42And this simple demo will give us the right mental model.
07:45If the moon was perfectly spherical in balance, it would spin like this,
07:49and one side wouldn't be more likely to line up with Earth than the other.
07:53But the moon's not perfectly spherical.
07:55Over time, that constant pull of Earth's gravity has caused it to be sort of slightly oblong,
08:00which we've sort of exaggerated a little bit here.
08:03But now you'll see, since the center of gravity of our moon doesn't exactly line up with
08:07the axis by which it rotates, like it was here,
08:11now, no matter how I spin it,
08:12it always happens to be that the heavy side of the moon always face the strongest source of gravity,
08:18which just so happens to be the center of the Earth.
08:21Well, guess what?
08:22Out in space, the actual moon's nearest source of gravity is still the center of the Earth,
08:26which means that the real moon will always have the same side pulled towards us,
08:31just like with our model.
08:32And that means every living thing on this planet that has ever looked up at the moon
08:36has always seen that same familiar face.
08:39And that will never change.
08:41Now, before we get to the last three,
08:43including answering how hot air balloons steer,
08:45if you're like me and you love the feeling of that aha moment when you learn something new,
08:49then I got great news for you,
08:51because packaging up that moment is why I created Crunch Land.
08:54It's Mark Grover's engineering box!
08:56Where you get a super fun toy every month in the mail that comes with a video
09:00where I teach you all the really cool physics that make it work.
09:03So if you want to have a ton of fun building your own toys
09:05while experiencing a bunch of those lovely aha moments at the same time,
09:09just visit CrunchLabs.com to learn more.
09:12And now I'm on top of a train to learn once and for all
09:15why do you land in the same spot if you jump inside the train,
09:18but not if you jump on top of the train?
09:20We'll get to the jump in a second, but first let's see if we can glean any clues from this
09:25small scale test
09:26with two Lego guys.
09:27One inside and one on top of the train,
09:30but they're in water.
09:32Now in a moment I'm going to pull on this string,
09:34which will move the train car forward.
09:36And I want you to think what will happen to each of the Lego guys.
09:39Ready? Here we go.
09:44Did you catch that?
09:45Now because this test was in water, you intuitively sort of had a sense the top guy would fall off
09:50and the bottom guy would be fine, right?
09:52Well, yeah, because the top guy would have to travel through all that still water
09:56and that would push back on him.
09:57Whereas inside the train car, he's enclosed.
10:00So everything in that car, including the water,
10:02is moving along with the train.
10:04So why would I show you this Lego guy standing on a model train in water?
10:08Because it's fundamentally no different than me standing on a real train in air!
10:13Because just like water, air is also a fluid, it's just not as dense as water.
10:17You can sort of see how it moves like a fluid here in this demonstration,
10:20showing hot air coming from a blow dryer.
10:22So when a train moves through the air, it's moving through a fluid.
10:26To visualize this, let's replace me with a pair of helium balloons.
10:29Once again, what do you think will happen here?
10:32Well, of course, the balloon inside is totally upright because just like in water,
10:35the air around that balloon is traveling with the train.
10:39Whereas on top, the balloon is being pushed back
10:41because it has to fight its way through all the stationary air.
10:44Just like the Lego guy on top being pushed back by all the water around the train in the tank.
10:49It's sort of like when you stick your hand out the car window
10:51and feel all that resistance from the wind.
10:53So going back to the jump, if you were able to visualize all the air molecules like this,
10:57you'd see how once we start moving inside the train when I jump,
11:00I get no pushback from all my air molecule neighbors
11:02because they're pretty chill and we're all traveling together.
11:05But on top of the train, it's a whole different story.
11:07I'm continuously bonking into a mad rush of trillions of air molecules every second as I whizz by.
11:13So if I jump now, it makes total sense why they would push me back and I would land in
11:18a whole different spot.
11:19For number six, here's an interesting thought experiment.
11:22Let's say you just bought a small two-seater airplane
11:24and you know the plane has to be traveling at 60 miles per hour on a still day to take
11:27off.
11:28So one day, you have the brilliant idea.
11:29Why not just create a much shorter runway using a plane-sized treadmill that can ramp up to 60 miles
11:35per hour,
11:36sending it into the air from your own backyard?
11:38But would that actually work?
11:40Well, let's run the experiment together with a small scale model.
11:43Now the way a plane typically takes off is it needs to reach a certain speed,
11:46which of course then requires a certain length of runway.
11:49So let's see what it is for this model airplane.
11:51Looks like it's 10 miles an hour after traveling 15 feet.
11:55Now here we've got our own version of our backyard treadmill runway
11:57that will ramp up to our 10 miles per hour takeoff speed.
12:00So let's see what happens.
12:01Make your prediction now if you haven't already.
12:04As we start the treadmill, we're going to spin the propeller more and more to keep it in the same
12:07spot
12:07so it stays in the camera frame.
12:09Okay, so we're at takeoff speed and sadly, we're not taking off.
12:14And at this point, we were suspicious maybe we measured something wrong.
12:17So we cranked it up way more and still nothing.
12:20So does that mean our backyard treadmill idea is a bust?
12:24Let's talk about what needs to happen for a plane to take off.
12:27Because planes need air flowing around their wings to create lift.
12:30Once again, it's sort of like sticking your hand out the car window.
12:33You don't feel any lift stopped at a red light, but the faster you go
12:36and the more air that flows by your hand, the more lift you feel.
12:39Which means no matter how fast our treadmill goes, there's still no air moving over the wings.
12:44We're stopped at a red light.
12:46And I think part of the confusion many people have with this experiment
12:48is they're comparing it to the vehicle they know best, which is a car.
12:51Because for a car, the ground is what gives you forward motion.
12:54But for a plane, it's the propeller that gives you forward motion.
12:59Here's a thought experiment to demonstrate that.
13:00Imagine you have a car and a plane on the most slippery ice ever invented.
13:05Because the car's engine uses the ground to get forward motion through the tires,
13:09no matter how fast you spin those tires, the car will not move an inch.
13:14The plane, however, is totally unaffected by the ice.
13:16Because the plane's wheels aren't powered at all.
13:19The forward motion for a plane comes from the propeller in the air.
13:23So it would take off with absolutely no problems.
13:26It wouldn't know the difference.
13:27So knowing that, let's adjust this experiment to give the plane a chance to move forward.
13:32Which means, of course, we're gonna need a bigger treadmill.
13:35So let's get our treadmill back up to that 10 miles per hour takeoff speed.
13:39And at this point, we're exactly back to where we started with the smaller treadmill.
13:43So to get the plane to move forward, let's just give it more power,
13:46causing it to move down our extended treadmill and take off at 10 miles per hour after 15 feet.
13:52Which, as you recall, is the same speed and distance as when we tested it with no treadmill at all.
13:58So would your brilliant backyard runway treadmill idea work?
14:01Absolutely. Assuming, of course, your backyard is the exact length of an actual runway.
14:07And now for our final engineering puzzle, we're back in this hot air balloon as I contend with my mild
14:12fear of heights.
14:13Okay, well, I haven't found a steering wheel, but it does seem pretty clear every time you add hot, we
14:19go high.
14:19Hot air balloons utilize some pretty straightforward physics, and the clue is right there in the name.
14:24Hot air is less dense than cold air. And we know from our Coke cans, less dense things will rise.
14:30So when Mateo does this and heats up the air in the balloon, he's basically diet coking us.
14:36And then over time, the air outside cools back down the air in the balloon, making it more dense,
14:40and therefore we start to sink down like regular Coke.
14:43So all you have control over is whether we go up or down.
14:46Correct.
14:46But you said you could land me wherever I wanted.
14:49That is also correct.
14:50But what if the wind's going that way, and I want to land over there?
14:54That's gotta be magic.
14:55Yeah, so what we'll do is we'll-
14:56Okay, pause here, because to understand Mateo's answer, you first need to understand two things.
15:00The first is that if the wind is blowing in one direction in your backyard,
15:04a thousand feet above that, it's almost certainly blowing differently.
15:07And a thousand feet above that, it will once again be totally different.
15:11So the question then becomes, how does Mateo know the wind's speed and direction at different heights?
15:16And that's where number two comes in, because it's someone's job every day to launch two balloons like this
15:21into the sky at noon and midnight London time.
15:23But this is done in a thousand locations all around the world, all at those same two exact times.
15:28And these balloons all have something called a radiosonde attached to them
15:31that measures things like altitude, pressure, temperature, and wind.
15:34Then they transmit the information back to the ground station, which gets fed into supercomputers.
15:39And that's the reason the wind and weather predictions can be so accurate.
15:42So 2,000 of these massive weather balloons go up every day,
15:45and they eventually pop and just land somewhere, never to be retrieved.
15:50Now because of this, computers can figure out the wind direction and speed at every level of the atmosphere as
15:55you go up.
15:56So on a given day, Mateo just checks the daily charts, and if these are the predicted wind directions and
16:00speed,
16:00and he wants us to land exactly right here, he just needs to raise, and then raise again,
16:05and then lower the balloon to catch the appropriate invisible rivers of air to land in the perfect spot.
16:11So hot air balloons don't steer, at least not in the normal way we think of.
16:15It's more of a tag team with mother nature.
16:17So in some ways, he's less of a magically steering pilot, and he's more of a weatherman.
16:22Which means the only real magic here is all the juicy knowledge I just wirelessly transferred
16:27through that screen from my brain into yours.
16:33This is Timmy, and he is a piano prodigy.
16:37He loves to practice, and his parents don't have to nag him to practice either.
16:41I love it.
16:42Do your thing, Timmy.
16:44But this is Katie, and she lost all interest in piano after Hot Cross Buns.
16:49Which means practice is a battle and a chore she doesn't want to do.
16:52And let's face it, Mom doesn't exactly want to hear either.
16:56Well, I got great news for you, Katie and Mom, because for the cost of one month of begrudging piano
17:00lessons and reactions like this,
17:02you can get an entire year of Crunch Slab's build box with reactions like this.
17:09And when the stoke level is this high, you're learning so much cool stuff even without realizing you're learning so
17:15much cool stuff.
17:17Because each month when the new toy arrives on your porch, and then you put it together alongside me,
17:21you're learning all the cool engineering principles that make it work.
17:24And this secret sauce of hiding the vegetables is the reason we've shipped millions of these boxes to converted parents
17:30around the world,
17:30saying this has totally unlocked a new level of resilience, curiosity, and passion in their kids.
17:36And there's even two options.
17:38You've got build box, where I teach them the basics through building a super fun toy for kids up to
17:4212 years old.
17:43And then there's hack pack, where we learn some coding with an awesome build-it-yourself robot for your teens.
17:49Or heck, even for you.
17:50Half of these things get shipped to adults.
17:52So if you want to go from being the household nagger to the household legend,
17:55watching a young mind discover a passion they didn't even know they had,
17:59go to crunchlabs.com or use the link in the video description,
18:02where we're currently giving away either one or two boxes free as an early spring special.
18:06Thanks for watching.
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