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  • For this video Lego sent me some of their Saturn V rocket sets.

  • But this isn't a video about space - I actually want to talk about the physics of how to keep

  • tall things from shaking, like how to keep skyscrapers from swaying too much in the wind,

  • or in an earthquake.

  • The taller something is, the more liable it is to sway back and forth, so to demonstrate

  • this, the Saturn V seemed by far like the best and coolest Lego set for the job (even

  • if not the most realistic).

  • If you don't your tall thing to sway, either because you're worried it'll fall apart

  • or you're worried it'll freak out the people inside, you could just make it stiffer

  • either by adding more stuff or using more rigid materialsthis isn't always the

  • most elegant solution, and gets expensive fast.

  • But there's another, clever solution.

  • From a physics perspective, a tall thing like a building is really an upside-down pendulum

  • - when it gets bent a little bit to one side, the building's natural stiffness pulls it

  • back the other way, and so on.

  • And there's a neat phenomenon that happens with pendulums when you attach two of them

  • together with a spring: they start swapping energy back and forthfirst one oscillates,

  • then both together, then the other has all the oscillations, then the first again, and

  • so on.

  • The same thing happens with solid blocks and springs, too, or any two oscillating things

  • that are coupled together.

  • In a perfect frictionless simulation this energy swapping will go on forever.

  • But in the real world there's friction and air resistance and the spring itself might

  • heat up, causing the objects to lose energy, and the oscillations dampen over time.

  • And this phenomenon is what you can use to 'discourage' your upside-down pendulum

  • from shaking in ways you don't want it to.

  • A big tall object is going to want to sway back and forth at its own natural frequency

  • of swaying (which depends on its height, weight, and stiffness).

  • If you then attach a smaller object to it that can sway to and fro, and add a little

  • bit of friction, then any time the big object trades its energy to the little object, the

  • little object loses the energy to friction rather than trading it back to the big object,

  • and this dampens out the big object's oscillations.

  • In practice, you normally put the little object inside the big object, which looks more like

  • this, but the principle is the same.

  • And you don't want to use just any old little object and any old spring.

  • It turns out that for a particular big thing, there's an optimal combination of weight

  • and friction and spring strength for the little object.

  • And only with your setup perfectly tuned will you get the fastest possible loss of energy

  • and the best slowing down of the shaking.

  • That's why this setup is called a “tuned mass damper” – “damperbecause it

  • dampens the swaying, andtunedbecause the little object is specifically tuned to

  • steal energy from this particular big thing's natural tendency to swing.

  • Which brings us to the Lego Saturn V rockets: in one of the rockets I've put a weighted

  • pendulum in the place the lunar module should go, while the other just has the weights stuck

  • in place.

  • If all goes well, and if I've done my math right, then when I bump the table, the rocket

  • with the pendulum should sway a lot less than the rocket without.

  • As you can see, the tuned mass damper does actually help! (though it's a bit subtle).

  • Here's a graph of the motion of the rocket in each case, which makes it more obvious.

  • And though tuned mass dampers definitely weren't used in the saturn V rocket in this way (since

  • this is where the lunar module went), they are used in skyscrapers and even in other

  • things that aren't buildings (though they're usually hard to notice).

  • For example, if you look closely at power lines, a lot of them have this little dumbbell

  • thing on them, which is a tuned mass damper that keeps the lines from shaking too vigorously

  • in the wind.

  • And tuned mass dampers have been used to reduce unwanted vibrations in airplane engines, formula

  • 1 racing cars, and audio speaker cones.

  • So there you gothe tuned mass damper, aka the physics of how to use little things

  • to stop big things from shaking!

  • As you've probably guessed by now, this video was sponsored by LEGO - I've been a huge fan

  • of LEGO for pretty much my entire life, spending hours and hours building and rebuilding all

  • sorts of LEGO projects as a child and teenager, and even more recently sneaking some into

  • MinutePhysics videos.

  • So I'm excited that LEGO wanted to full-on sponsor a video.

  • The Saturn V sets are super cleverly designed and really fun to build, and of course I'm

  • a big Apollo fan and I love how the rocket actually stages like the real thing.

  • Anyway, the folks at LEGO want me to point you to their online and physical stores which,

  • naturally, have the largest collection of lego sets, and also allow you to find and

  • buy individual pieces a la carte for custom projects, get expert lego help, and with LEGO

  • VIP you can get early access to new LEGO sets.

  • Thanks so much to LEGO for sponsoring this video and filling my childhood with creative

  • delight.

For this video Lego sent me some of their Saturn V rocket sets.

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How To Stop Structures from SHAKING: LEGO Saturn V Tuned Mass Damper

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    Summer posted on 2020/07/30
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