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The wheels on the bus go.. (Posted on 2003-06-23) Difficulty: 4 of 5

If you drew a dot on the edge of a wheel and traced the path of the dot as the wheel rolled one complete revolution along a line, then the path formed would be called a cycloid (shown below), combining both forward and circular motion.

If a wheel of radius 1 traces out such a path, what is the length of the path formed by one complete revolution?

  Submitted by DJ    
Rating: 4.3571 (14 votes)
Solution: (Hide)
8 units

Contrary to the diagram in the problem, place the center of the wheel at (0,0) and draw the starting point at (0,1).
Let t be the distance the center of the wheel has moved from (0,0). Then, we have:
x=t+sin t
y=cos t

Taking the derivitives:
dx/dt=1+cos t
dy/dt=-sin t
The change in arc length can be defined as √((dx/dt)²+(dy/dt)²).

So the total arc length is the integral from 0 to 2π of √((dx/dt)²+(dy/dt)²).

After a few steps this integral becomes:
(√2)(√(1+cos t))

Multiply by [√(1-cos t)/√(1-cos t)] and the integral becomes:
(√2(sin t))/(√(1-cos t))

Let u=cos t and the integral becomes:
(√2)/(√(1-u))

Integrating this you get:
(√2)(2)(√(1-u))

The bounds are 0 to 2π, so the total arc length is:
(√2)(2)(√2+√2)
=(2√2)²
=8

friedlinguini offers a slightly different solution (with the same result) here and here.

Comments: ( You must be logged in to post comments.)
  Subject Author Date
Puzzle AnswerK Sengupta2024-02-13 02:54:51
AnswerK Sengupta2008-03-20 09:53:32
Nice ProblemR.Kesavan2004-11-24 14:45:29
Some Thoughtshalf way thereLarry2004-02-21 14:26:04
guess'n i think.dorkdork2003-08-02 23:46:37
re: SolutionBryan2003-06-23 09:35:02
re: initial reactionlevik2003-06-23 07:46:13
Solutionre: Solutionfriedlinguini2003-06-23 06:30:43
Some ThoughtsSolutionfriedlinguini2003-06-23 05:23:20
initial reactionHank2003-06-23 04:48:12
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