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A.1 · 3 · Enrichment · mathematics and Theory of Knowledge

Zeno: is all motion an illusion?

An ancient paradox, settled by a series that adds to a finite number.

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Hook

To walk from A to B you must cover half the distance, then half of what is left, then half again, forever. So how does anyone arrive?

Zeno of Elea
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The paradox

“All motion is an illusion. Nothing moves.”

StepDistance still to cover
1st step½ of the way
2nd step¼ more
3rd step⅛ more
…∞ steps∞ time?always some of it left
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A convergent series…

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After 3 terms of ½ + ¼ + ⅛ + …, the total is…

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What is wrong with "infinite steps need infinite time"?

Apply

In two sentences, explain how a convergent series settles Zeno’s paradox.

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Remember earlier lessons
From lesson A.1 · 2

Frequency is…

From lesson A.1 · 1

Motion is relative because…

Summary card

Key points

  • • Zeno claimed infinite steps mean infinite time.
  • • ½ + ¼ + ⅛ + … converges to 1.
  • • Convergent: finite. Divergent: infinite.

Formulas

  • Sₙ = 1 − (½)ⁿ

Key terms

Convergent
:
the sum is finite
Divergent
:
the sum grows without limit

Common errors

  • • Thinking infinitely many terms must sum to infinity.
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