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My math teacher has struck again.

Here's his newest riddle:

Today I went for a normal walk of 3000 meters.
One of my feet had to move exactly 3000 meters.
However, the second foot moved 3100 meters.

Can you justify how did that happen?

Edit:

I am not sure how this question has been selected as too broad.
It clearly only has a tag and the correct answer has been given and accepted in a fast way. If people decided to answer it using it doesn't mean that the question is too broad, it means that posters just want to get the "funny" comments and a "+1".

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    $\begingroup$ Man, I wish this had a lateral thinking tag on it... I have this great answer about a guy attempting to step off a sidewalk and stepping back again numerous times due to traffic! $\endgroup$
    – Phylyp
    Commented Apr 27, 2018 at 12:53
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    $\begingroup$ I like how he seems to downplay his obsessive foot step measurement practices, causing all sorts of dysfunctions in his day-to-day life, by asserting that this is just “a normal walk” $\endgroup$
    – kedarguru
    Commented Apr 29, 2018 at 0:00
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    $\begingroup$ No, actually, the choice of the word 'normal' more or less uniquely defines the correct solution to this riddle. $\endgroup$
    – Glorfindel
    Commented May 2, 2018 at 6:58
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    $\begingroup$ @PaulKaram No, see my answer below: the word must be "normal" because that describes the walk very precisely; it makes the question a lot less ambiguous. That is the "lateral thinking" part of the question... to figure out that the word "normal" is not used in the... heh... normal sense it is used in common parlance. :) $\endgroup$
    – MichaelK
    Commented May 2, 2018 at 7:12
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    $\begingroup$ The excessive answers, many of which are definitely (and even admittedly) lateral-thinking despite the question being solidly grounded on mathematics, are inappropriate. If you're not making a good faith effort to answer the question as posed, please restrain your urge to indulge in posting "clever", "funny" or "unique" solutions. We routinely delete responses that rely on lateral thinking when no such tag was indicated, as they are Not an Answer to the actual puzzle. Most of these answers are likewise Not an Answer. They will be deleted soon. $\endgroup$
    – Rubio ♦
    Commented May 3, 2018 at 21:13

8 Answers 8

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Based on @TwoBitOperation's answer

and assuming his feet are 25 centimeters apart, if he walks $n$ circles with a radius of $r$ meters in a single direction, one foot walks $2 \pi r n$ meters, and the other one $2 \pi (r + 0.25) n$. The difference, $\pi n / 2$ is 100 meters, so $n = 200 / \pi \approx 63.6$. The value of $r$ is then determined from $2 \pi r n = 3000$ so $2 r = 15$ and $r$ = 7.5 meters.

Whether these

63.6 turns around e.g. a fountain or pond with a 15m diameter constitute a "normal walk" remains an open discussion.

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    $\begingroup$ Very smart way of getting a variable $n$ into it :-) $\endgroup$
    – Phylyp
    Commented Apr 27, 2018 at 12:57
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    $\begingroup$ Oh, anything constituting $n$ and circles is normal for a Mathematician. $\endgroup$
    – Sid
    Commented Apr 27, 2018 at 13:05
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    $\begingroup$ It's not necessary for the path to be a uniform circle as long as the total amount turn is right. A 3 km straight walk where you make a little circle every 50 m (say, to look behind you) would also do. $\endgroup$
    – hmakholm left over Monica
    Commented Apr 27, 2018 at 15:06
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    $\begingroup$ Does the same apply at certain latitudes simply by walking East-West? $\endgroup$
    – Sentinel
    Commented Apr 28, 2018 at 20:32
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    $\begingroup$ If he is positioned half-way between his feet (which sounds normal to me), won't he move 3050 metres while one foot moves 3000 metres and the other foot moves 3100 metres? $\endgroup$
    – CJ Dennis
    Commented Apr 30, 2018 at 5:25
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This person has an artificial left leg, and they were just minding their own business walking down the sidewalk when...WHAM!...a car comes along and takes out their left leg and sends it flying 100 meters!

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    $\begingroup$ Wait this has gotta be the funniest answer I've seen in a long time :P +1 $\endgroup$
    – NL628
    Commented Apr 27, 2018 at 15:17
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    $\begingroup$ Thanks — Just had an out-loud laughing fit, at work, because of this answer :) $\endgroup$
    – Caleb
    Commented Apr 27, 2018 at 17:09
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    $\begingroup$ Uh oh, funny answer. Counting down the minutes before a buzzkill mod with delete privileges comes around and obliterates it. $\endgroup$
    – user1717828
    Commented Apr 27, 2018 at 21:06
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    $\begingroup$ Mod Buzzkill, reporting for duty. Actually this made me laugh out loud as well, and—in a rare fit of good humor—I'm pretending I didn't see this. Carry on! $\endgroup$
    – Rubio ♦
    Commented Apr 29, 2018 at 1:26
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    $\begingroup$ The left leg doesn't have to be artificial. :P $\endgroup$
    – JRN
    Commented May 2, 2018 at 5:53
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You walked a full circle with diameter ~1000m

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    $\begingroup$ I thought the same, however this would only work if his feet were ~16m apart... $\endgroup$
    – Fifth_H0r5eman
    Commented Apr 27, 2018 at 12:46
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    $\begingroup$ maybe they are do you have a problem with that? $\endgroup$
    – L_Church
    Commented Apr 27, 2018 at 12:47
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    $\begingroup$ If pi * d = 3000, then assuming the two feet are 1 meter apart (an exaggeration, I know), then pi * (d + 1) = (pi * d) + pi, so the extra distance covered is only about 3 metres. $\endgroup$
    – Phylyp
    Commented Apr 27, 2018 at 12:47
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    $\begingroup$ Also see this puzzle which illustrates the same fallacy in another way. $\endgroup$
    – Phylyp
    Commented Apr 27, 2018 at 12:49
  • $\begingroup$ This is a good point @Phylyp, and seeing as this is tagged 'mathematics' I'm thinking I must be wrong $\endgroup$
    – TwoBitOperation
    Commented Apr 27, 2018 at 13:06
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Well, I struggle a bit to explain my point, but

If you go back and forth on a straight line of 30 meters one hundred time and put the same foot half a meter meter ahead each time you reach the end of the line and have to turn around, your body will move 3000 meters, one of your foot will move 3000 and the other one 3100

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  • $\begingroup$ Step one meter ahead with one foot, then bring it back - it's moved two meters, not one. So with your example, the first time down the path you step 31 meters with one and 30 with the other. On every subsequent lap you're doing 32 and 30 (as one foot travels an extra meter at the start and end each time). After 100 laps your feet will have traveled 3199 and 3000 meters. You could adjust the number of laps you take the extra step... $\endgroup$
    – sirjonsnow
    Commented Apr 27, 2018 at 14:07
  • $\begingroup$ then go for 50cm instead of one meter :D not a typical walk but hey it fills criterias! $\endgroup$
    – RiddlerNewComer
    Commented Apr 27, 2018 at 14:35
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    $\begingroup$ The Ministry of Silly Walks would like to have a word $\endgroup$
    – nzaman
    Commented Apr 28, 2018 at 13:09
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You were

running down a spiral staircase. this keeps one foot only mostly moving up and down, whiclst the outside foot also has circular movement. The circle is small enough that 100m is much more resonable

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  • $\begingroup$ You should hide your answer with the spoiler markup, and explain why you this this is the proper answer. $\endgroup$
    – Herb
    Commented Apr 27, 2018 at 16:02
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    $\begingroup$ Welcome to Puzzling.SE! Feel free to take the tour and visit the help center to learn more about the site. $\endgroup$
    – F1Krazy
    Commented Apr 27, 2018 at 16:13
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    $\begingroup$ I like this answer, because it also incorporates the geometric meaning of "normal", ie. perpendicular to the plane of the ground. $\endgroup$
    – Chromatix
    Commented Apr 27, 2018 at 21:12
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    $\begingroup$ You should add an elevator every once in a while to reset your position, otherwise you may be on a nearly 1000 meters high staircase. $\endgroup$
    – Cœur
    Commented Apr 28, 2018 at 12:26
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    $\begingroup$ 300m of stair might be enough, still quite a workout. $\endgroup$
    – Jasen
    Commented Apr 29, 2018 at 9:42
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Your professor went on a walk where he paced back and forth, as professors often do. If we assume the professor has a spacing of roughly 16 centimeters (~15.9154943) apart for his feet while walking, and that he always stops on the same foot, he will pivot on that foot, and need to swing his other foot in a half circle arc (Pi*Radius) before continuing in the other direction. At this spacing his second foot will move an extra .5 meters each time he switches directions. He will need to switch directions 200 times for the second foot to go 100 meters further. If we assume he doesn't pivot when starting, but pivots and turns one final time at the end before deciding to stop, he will need to pace back and forth a distance of 15 meters (3000/200) to have one foot go 3000 meters and the other 3100 meters.

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  • $\begingroup$ Welcome to Puzzling.SE! Make sure to take the tour! $\endgroup$
    – NetJohn
    Commented Apr 27, 2018 at 20:15
  • $\begingroup$ This answer has already been given. $\endgroup$
    – Peregrine Rook
    Commented Apr 30, 2018 at 22:44
  • $\begingroup$ The closest answer has the person taking an extra step with one foot and then walking back, which is different from moving the foot in an arc at each turn around. $\endgroup$
    – A. Clark
    Commented May 1, 2018 at 16:02
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The amount of distance traveled by each foot is determined not only by the amount of distance it covers in the forward direction, but also the amount of distance it covers in the upwards and sideways directions. Most people don't have a perfectly symmetrical gait, so it's unlikely the exact path each foot takes on its way forward is the same. One foot probably steps a little higher or a little wider than the other, and over the course of a great many steps, these differences can add up. If one foot took a slightly longer step than the other, this would have an even greater effect. The foot with the shorter step would have to be raised more often for a given forward distance covered, and each time it is lifted, that adds a little upward distance to that foot's travel.

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  • $\begingroup$ I like the concept! It would work very well for someone who, say, couldn't bend one of his knees and has to swing that leg round. $\endgroup$
    – AndyT
    Commented Apr 30, 2018 at 9:21
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The key to the question is "Normal"

Today I went for a normal walk

A "normal" in geometry is not something "common" or "ordinary". A geometrical "normal" is always perpendicular to something else.

Let us say that this something is...

...a line that extends from a fixed point, to the walker.

If the walker then walks along the "normal" of this thing, they will...

...always walk in a circle. The point is the center of the circle. The line in question is the radius of the circle. Walking along the normal of a radius of a circle means you walk along the circle itself.

If you walk this way, one foot will move further than the other.

Now let us do the Maths:

Let us assume that the distance between the walker's feet are, well, one foot, 30 cm, or 0.3 meters. Let us assume they went $x$ number of laps around the circle. So the distances walked is $(r + 0) \cdot x \cdot 2\pi = 3000$ for the inner foot, and $(r + 0.3) \cdot x \cdot 2\pi = 3100$ for the outer.

Divide one by the other and we get:

$\frac{(r + 0) \cdot x \cdot 2\pi}{(r + 0.3) \cdot x \cdot 2\pi} = \frac{3000}{3100} \Rightarrow$

$x\cdot 2\pi$ cannot be zero so we can cancel that factor out

$\frac{r}{r + 0.3} = \frac{30}{31} \Rightarrow$

$31\cdot r = 30\cdot r + 9$

$r = 9$

Hence...

...they walked along a circle that had a radius of 9 meters, with the inner foot directly on the circle. I leave it to the reader to figure out how many laps it was.

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    $\begingroup$ (But, other than specific figures, how is this different from the accepted answer?) $\endgroup$
    – Rubio ♦
    Commented May 4, 2018 at 14:51
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    $\begingroup$ @Rubio The maths is the same but this answer comments on the "normal" part and explains why it must be a circle. $\endgroup$
    – MichaelK
    Commented May 4, 2018 at 14:56
  • $\begingroup$ I suppose. But the interpretation of "normal" in this sense isn't even necessary to arrive at this answer (the accepted answer didn't need to rely on it). Beyond that, the basic math is the same, and the substance of the answers is the same as well, making this answer really a dup with an extra interpretation tacked on that may or not have been intended by the OP. If all you are really adding is that interpretation, that would probably be better done by leaving a comment on the accepted answer. $\endgroup$
    – Rubio ♦
    Commented May 4, 2018 at 15:00
  • $\begingroup$ It may not be necessary but it was included in the riddle, so I wanted to talk about why it is there. $\endgroup$
    – MichaelK
    Commented May 4, 2018 at 15:13

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