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A lucky escape

Posted by Chris on December 11, 2010 – 7:12 pm

Two boys walking in the woods decided to take a shortcut through a railroad tunnel. when they had walked two-third of the way through the tunnel, their worst fears were realized. A train was coming from the opposite direction, nearing the tunnel entrance. The boys panicked and each ran for a different end of the tunnel. Both boys ran at the same speed, 10 miles per hour. Each boy escaped the tunnel just at the instant that the train would have squashed them into the rails.

Assuming the trains’ speed was constant, and both boys were capable of instantaneous reaction and acceleration, how fast was the train going ?

I have solved this one. If I had done that earlier, I probably wouldn’t have bothered posting it.


This post is under “Fun Physics” and has 10 respond so far.
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10 Responds so far- Add one»

  1. 1. nimble11 Said:

    30 mph! The boy running the longer length will still have 1/3 the distance to go when the other boy escapes. The train has to go 3 times the distance as the remaining boy in the same time span.

  2. 2. Chris Said:

    Hi nimble11. Your method of deducing the result is slicker than mine. Nicely done, and of course 30 mph is the right answer :)

  3. 3. Karl Sharman Said:

    30 mp…… grrr

  4. 4. Al Gelman Said:

    I did it by brute force (algebra).
    2/3*L=10(L/V + L/30)
    V = 30

  5. 5. mreije Said:

    V(train)=5

  6. 6. vedsar kushwaha Said:

    velocity of train must be 30 miles per hour…..
    let there are two boys, say A,B.
    A has to cover 1/3rd distance and takes time t.In the mean while train enters the tunnel, and boy B travels 1/3rd distance(Know he has two travel 1/3rd more).
    Therefore if boy B takes t time, he covers 10t distance and the train covers 30t distance(since B starts with A, A travels 10t, remaining 20t traveled by B). hence speed of train is 3 times the speed of boy…
    i.e speed of train is 30miles/hours

  7. 7. Noah Said:

    20 mph
    i say
    what do you thiink?

  8. 8. Chris Said:

    Hi Noah. Why do you say 20 mph?

  9. 9. Your name Said:

    its simple its 30mph

  10. 10. parth Said:

    15mph is ur speed
    bt i not conform what u think…???

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