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  • Balancing a pencil or any other tough, heavy object on its tip,

  • is quite a precarious task.

  • It is easy to see why tough heavy object are unstable.

  • They justwanna fall down.

  • Any little nudge, will put it off balance,

  • with gravity, happy to pull its center of mass even more off balance

  • And the more off balance, it is the more gravity pulls in too!

  • Physicist calls this kind of tipping an...Inverted Pendulum.

  • Because its kind of, likean upside-down pendulum.

  • And careful mathematical analysis of Inverted Pendulums...will tell you the obvious,

  • theyre reallyunstable.

  • In fact, this equation predicts that

  • if a perfectly balanced pencil with a perfectly-sharped tip,

  • will do get nudge, off balance, by just one ten thousandth of the width of an atom.

  • It would take only three seconds, to fall over...

  • There are indeed acrobats, who can balance poles, with other acrobats on top.

  • And the robots, they can balance and, catch and throw, upright sticks!

  • And the people who make... seemingly impossible, artistic, rock stacks.

  • But in all these cases,

  • keeping the objects balance, either requires an active stabilization effort,

  • Or else, carefully taking advantage of the fact that

  • the balancing point...isn't actually...a point!

  • So, the question is this:

  • Can you balance a pencil sharpened to a perfect, single, atom-wide tip, on its tip?

  • Without active stabilization?

  • Well, to start, you better get rid of the air bumping in besides the pencil.

  • Since the kinetic energy of a typical oxygen molecule,

  • is enough to destabilize the pencil.

  • Remember, if the pencil gets off center by even ten thousandth of the width of an atom,

  • it would fall down in, under three seconds.

  • And you better cool the pencil and table down as close to absolute zero, as you can...

  • since the thermal jiggling of their atoms when they're hotter, are similarly jiggle the pencil off balance.

  • But even then, matter add very small scales has spread out wape like nature

  • which means that you cannot have both pencil perfectly upright,

  • and not moving at the same time.

  • And while this "quantum effect" is never a real life inhibitor of the precarious pencil balancing?

  • It means, you never even could succeed...

  • However, it is still fun to see, how long you can keep a pencil balanced on your fingertip.

  • The shorter the pencil, the harder it is.

  • My best time for a standard nineteen centimeter pencil

  • was one point three seconds.

  • Do you think you can beat that?

  • This minute physics video is brought to you by

  • one of the leading providers online, audio books, and all types of media,

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  • I'd like to recommend "The Tipping Point" by Malcolm Gladwell this week.

  • It is a book about exponential increases and spreads of trends,

  • social trends, academic, that sort of thing,

  • and...the mathematics of, the mathematics of... balancing an object, on its...tip,

  • so...anyway,"The Tipping Point", by Malcolm Gladwell,

  •, supports minute physics.

  • Without them, we won't be able to do... what we do!

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Balancing a pencil or any other tough, heavy object on its tip,


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B1 中級

鉛筆のバランスをとるには、どのくらいの時間が必要ですか? (How Long Can You Balance a Pencil?)

  • 1998 94
    Go Tutor に公開 2021 年 01 月 14 日