B2 中上級 3836 タグ追加 保存
In the third act of "Swan Lake,"
the Black Swan pulls off a seemingly endless series of turns,
bobbing up and down on one pointed foot
and spinning around, and around, and around 32 times.
It's one of the toughest sequences in ballet,
and for those thirty seconds or so,
she's like a human top in perpetual motion.
Those spectacular turns are called fouettés,
which means "whipped" in French,
describing the dancer's incredible ability to whip around without stopping.
But while we're marveling at the fouetté, can we unravel its physics?
The dancer starts the fouetté by pushing off with her foot to generate torque.
But the hard part is maintaining the rotation.
As she turns,
friction between her pointe shoe and the floor,
and somewhat between her body and the air,
reduces her momentum.
So how does she keep turning?
Between each turn, the dancer pauses for a split second and faces the audience.
Her supporting foot flattens,
and then twists as it rises back onto pointe,
pushing against the floor to generate a tiny amount of new torque.
At the same time, her arms sweep open to help her keep her balance.
The turns are most effective if her center of gravity stays constant,
and a skilled dancer will be able to keep her turning axis vertical.
The extended arms and torque-generating foot
both help drive the fouetté.
But the real secret and the reason you hardly notice the pause
is that her other leg never stops moving.
During her momentary pause,
the dancer's elevated leg straightens and moves from the front to the side,
before it folds back into her knee.
By staying in motion, that leg is storing some of the momentum of the turn.
When the leg comes back in towards the body,
that stored momentum gets transferred back to the dancer's body,
propelling her around as she rises back onto pointe.
As the ballerina extends and retracts her leg with each turn,
momentum travels back and forth between leg and body,
keeping her in motion.
A really good ballerina can get more than one turn out of every leg extension
in one of two ways.
First, she can extend her leg sooner.
The longer the leg is extended, the more momentum it stores,
and the more momentum it can return to the body when it's pulled back in.
More angular momentum means she can make more turns
before needing to replenish what was lost to friction.
The other option is for the dancer
to bring her arms or leg in closer to her body
once she returns to pointe.
Why does this work?
Like every other turn in ballet,
the fouetté is governed by angular momentum,
which is equal to the dancer's angular velocity times her rotational inertia.
And except for what's lost to friction,
that angular momentum has to stay constant while the dancer is on pointe.
That's called conservation of angular momentum.
Now, rotational inertia can be thought of
as a body's resistance to rotational motion.
It increases when more mass is distributed further from the axis of rotation,
and decreases when the mass is distributed closer to the axis of rotation.
So as she brings her arms closer to her body,
her rotational inertia shrinks.
In order to conserve angular momentum,
her angular velocity, the speed of her turn,
has to increase,
allowing the same amount of stored momentum
to carry her through multiple turns.
You've probably seen ice skaters do the same thing,
spinning faster and faster by drawing in their arms and legs.
In Tchaikovsky's ballet, the Black Swan is a sorceress,
and her 32 captivating fouettés do seem almost supernatural.
But it's not magic that makes them possible.
It's physics.


【TED-Ed】The physics of the "hardest move" in ballet - Arleen Sugano

3836 タグ追加 保存
Bruce Lan 2016 年 4 月 3 日 に公開
  1. 1. クリック一つで単語を検索


  2. 2. リピート機能


  3. 3. ショートカット


  4. 4. 字幕の表示/非表示


  5. 5. 動画をブログ等でシェア


  6. 6. 全画面再生


  1. クイズ付き動画


  1. クリックしてメモを表示

  1. UrbanDictionary 俚語字典整合查詢。一般字典查詢不到你滿意的解譯,不妨使用「俚語字典」,或許會讓你有滿意的答案喔