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Subject: "a walking man and a running dog"     Previous Topic | Next Topic
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Jules4Rog
Member since Nov-2-01
Nov-02-01, 06:40 AM (EST)
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"a walking man and a running dog"
 
   A man is taking his dog for a walk. They set off together for a waterfall 10 miles away. They set off exactly together and the man walks at a steady 4 miles an hour. The dog runs off at 8 miles an hour and when it reaches the waterfall it turns round and runs back to the man. When it reaches the man it turns round and runs back to the waterfall and so on until the man and dog arrive at the waterfall. How far has the dog run in total? At first I said 20 miles because it's running twice as fast as the man is walking and so must have gone twice as far in the same length of time, but then Roger looked at me and sid "Are you sure?" which left me doubting, so does anyone know if I'm right or not?


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  Subject     Author     Message Date     ID  
a walking man and a running dog Jules4Rog Nov-02-01 TOP
  RE: a walking man and a running dog alexb Nov-03-01 1
  RE: a walking man and a running dog HS Greer Nov-07-01 2
  RE: a walking man and a running dog iliaden Apr-02-06 3
     RE: a walking man and a running dog alexb Apr-03-06 4
         RE: a walking man and a running dog Zeno Apr-03-06 5
             RE: a walking man and a running dog Novice Apr-17-06 6
     RE: a walking man and a running dog JonJonBinks1 May-03-06 7
         RE: a walking man and a running dog iliaden May-21-06 8

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alexb
Charter Member
1845 posts
Nov-03-01, 02:39 PM (EST)
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1. "RE: a walking man and a running dog"
In response to message #0
 
   Have a look at

https://www.cut-the-knot.com/arithmetic/999999.html

The second half of the page is very relevant to your question. Not that the first half is entirely useless.


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HS Greer
guest
Nov-07-01, 09:27 PM (EST)
 
2. "RE: a walking man and a running dog"
In response to message #0
 
   This is another version of the problem solved by Von Neuman.
20 miles is correct.
The man walks 2.5 hours to go the 10 miles.
In 2.5 hours, at 8 miles per hour, the dog runs 20 miles.
HS Greer


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iliaden
Member since Aug-14-05
Apr-02-06, 10:23 PM (EST)
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3. "RE: a walking man and a running dog"
In response to message #0
 
   whenever I encounter such a problem, I always answer 20 miles, yet there is one point on which I am not certain:

when the distance between the man and the waterfall is so small that the dog could fit between them, couldn't the dog simply stand still, while touching at both the man and the waterfall, thus running at an infinite speed since it reaches both of it's targets at the same time. Then I guess the answer would be that the dog ran an infinite distance.

I perfectly understand that this does not make a lot of sence, yet could you explain me why am I wrong?


thanks

Ilia Denotkine


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alexb
Charter Member
1845 posts
Apr-03-06, 04:07 PM (EST)
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4. "RE: a walking man and a running dog"
In response to message #3
 
   You are not wrong. The mathematical problem is idealized, that's all. It is formulated and solved under the assumption that the dog is a geometric point.

Without an abstraction of this sort, even the famous Greek paradoxes would not make sense. The moment the distance between the turtoise and Achilles becomes smaller than the latter's foot length, he will simply step on the turtoise and end the competition. For 2000 years, however, people pondered over the paradox which was based on the assumption that the turtoise and Achilles are able to make an infinite number of steps.


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Zeno
guest
Apr-03-06, 09:18 PM (EST)
 
5. "RE: a walking man and a running dog"
In response to message #4
 
   >You are not wrong.

I disagree. Claiming that a dog that is standing still is "running at an infinite speed" is obviously wrong.


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Novice
guest
Apr-17-06, 08:46 PM (EST)
 
6. "RE: a walking man and a running dog"
In response to message #5
 
   >I disagree. Claiming that a dog that is standing still is "running at an infinite speed" is obviously wrong.


I ain't any wiz but I think relativity is the factor. From man point of view, dog is coming to him and vice-versa, I guess.


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JonJonBinks1
guest
May-03-06, 11:31 PM (EST)
 
7. "RE: a walking man and a running dog"
In response to message #3
 
   >when the distance between the man and the waterfall is so
>small that the dog could fit between them, couldn't the dog
>simply stand still, while touching at both the man and the
>waterfall, thus running at an infinite speed since it
>reaches both of it's targets at the same time. Then I guess
>the answer would be that the dog ran an infinite distance

Even if the dog was going at an infinite speed it wouldn't be moving and therefore have a sspeed of zero.
Infinity(speed)x 0(distance)=zero

I also agree with that guy who said the problem should be inturpurated as the dog and man were both geometric points, in which case there would be no way the dog could be travelling at an infinite speed.


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iliaden
Member since Aug-14-05
May-21-06, 11:29 PM (EST)
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8. "RE: a walking man and a running dog"
In response to message #7
 
   >Even if the dog was going at an infinite speed it wouldn't
>be moving and therefore have a sspeed of zero.
>Infinity(speed)x 0(distance)=zero

this is true, yet in our case, the speed is lim s
s→ ∞
and the distance is lim d
d→0

thus the speed is ALMOST infinite, and the distance is ALMOST zero.


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