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Two trains and a bird!

  1. Jan 30, 2009 #1
    I really thought I could have found this on the search function but hey, here goes!

    1. The problem statement, all variables and given/known data
    Two trains, each having a speed of 26 km/h, are headed at each other on the same straight track. A bird that can fly 60 km/h flies off the front of one train when they are 66 km apart and heads directly for the other train. On reaching the other train it flies directly back to the first train, and so forth. (We have no idea why a bird would behave in this way.) What is the total distance the bird travels before the trains collide (in meters)?

    2. Relevant equations
    average velocity=total displacement/total time

    3. The attempt at a solution
    I worked on this quite a few times, and I first solved for the velocites of the trains. Then I found out when they'd hit collide. My distance came out to be 60,000.00001. Is this correct?

    Just a bit of backgroud info on me! I'm a computer engineering student (and a girl!) and I'm having a hard time in calculus II and physics 201 at my school. I go to tutoring and get as much extra help as my schedule allows! I hope to learn a lot from this forum and succeed this semester! Happy to meet all of you!
  2. jcsd
  3. Jan 30, 2009 #2


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    First of all too much precision.

    None of your other variables are given with that precision.

    Second, how many hours until they collide? What is their relative velocities? How far apart is given. So ... how long is that?
  4. Jan 31, 2009 #3


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    lauriecherie: Your current answer is incorrect. You are given the distance between the trains. Can you show your work for how you computed the relative velocity of the two trains? After that, you could use your relevant equation to compute the time when the two trains collide, right?
  5. Feb 1, 2009 #4
    Well my initial answer was incorrect. To get the correct answer, I took half of 66 km (33 km) and figured out at 26 km/hr how long it would take for each train to reach the collision point. A x axis really helped me on this one. I drew the collision point as x=0 and one of the trains at -33km and the other at +33 km. So I divided 26 by 33 and got a little over an hour. So if the bird flies at 60 km/hr, I found out how far can it fly in that amount of time it takes before the two trains collide. My correct answer came out to be like 76.15 km. I verified it with someone else's response on this thread, who I guess would up changing his mind and deleting it. He/She had put 76.05 km but mine was different from his cause I did no rounding on my initial quantities. Thanks for all the inputs! I really had over anaylzed this problem! Your input was appreciated greatly!
  6. Feb 1, 2009 #5


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    Nice work. Your answer is correct (although the problem statement asks for the final answer in meters). Also, the unit symbol for hour is spelled h, not hr. See NIST for the correct spelling of any unit symbol.
  7. Feb 1, 2009 #6
    Yea, that was me lol. I am new to the forums and just found out that you're not allowed to post only answers ;)
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