1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Time, speed, Distance

  1. Jun 26, 2010 #1
    1. The problem statement, all variables and given/known data

    A bird, 720 km away from a train flies towards it at a speed of 120km/h. As soon as it reaches the train, it turns back towards the wall (starting position). The train travels at a speed of 15m/s towards the starting position of the bird. How much distance towards the wall will the bird cover before the train reaches it.

    3. The attempt at a solution

    15m/s= 54km/h

    Thus the train takes 720/54=13.33 hours to reach the wall.

    The relative velocity of the bird wrt train when travelling towards it is 54+120=174km/h

    The relative velocity when travelling away from the train is 120-54=66km/h

    Ratios of velocities (towards:away)=29:11

    Thus the ratio of time taken in travelling towards the train to away from the train =11:29

    Thus, the bird spent a total of [tex]\frac{29}{29+11}\times 13.33[/tex] hours travelling away from the train.

    The velocity of the bird travelling away from the train is 120km/h

    Thus, the distance travelled by the bird away from the train is [tex]13.33\times \frac{29}{40} \times 120=1160 km[/tex]

    Which is the wrong answer. The solution is 800 km. Why?
  2. jcsd
  3. Jun 26, 2010 #2
    I don't see where you computed at what point the (extremely fast) bird and the train meet. Relative velocities are unimportant. The 800 km seems to be how far beyond the wall the bird will get, since it's faster than the train.
  4. Jun 26, 2010 #3
    The bird flies towards the train and as soon as it meets the train, it turns back and flies at the wall. 800 km is the total distance covered by the bird while its flying away from the train. For example, the distance covered in the first run is [tex]\frac{720}{120+54}\times 120[/tex] km (towards the train).
  5. Jun 26, 2010 #4
    It is bad reasoning. Imagine that a bird has the same velocity as the train. Relative velocity toward the train is 240km/h, toward away - 0km/h! But in this case time to fly toward away has to be infinity. It is clear that it is not true - bird back starting point together with the train!

    Now imagine that bird fly ONLY ONCE toward the train and back. What is the ratio of time to fly toward train to toward back? Remember that there is no matter where bird meet the train.

    Last edited: Jun 26, 2010
  6. Jun 26, 2010 #5
    Isn't the correct answer 607 km past the wall, and not 800?
  7. Jun 26, 2010 #6
    No, 800 is right.

  8. Jun 27, 2010 #7
    Ok. I see where I've made the mistake. The velocity towards the wall is 120 km/h and towards the train is 174km/h.

    Ratio of time taken flying towards the train to away from the train is 120:174 (=20:29)

    Total time spent flying away is [tex]\frac{13.33\times 29}{49}[/tex]

    Speed of the bird is 120km/h

    This gives a distance of 946.77 ...
  9. Jun 27, 2010 #8
    Forget about the train! :smile:. There is no train - train sets only the rendezvous points.

    Your problem looks like: "There is a bird flying from A to B and then backward. How many time it is flying back?".

    Answer is simply: half of the time, isn't it?

    Then the bird flying again from A to C and backward... the same. A -> D -> A? The same. In all cases half of the time it is going toward point of meeting and half of the time backward. No matter where that points are.

    So, no matter train speed and relative speed. Train sets only B, C, D..... turning points.

    Train is nessecary only to calculate total time of flying. Half of that time bird is going toward "away from train". And you know velocity :smile:.

    of course, the speed of the bird must be greater than the speed of the train. In order to fly earlier than the train to the starting point.

    BTW... bird flying for over 13 hours with speed 120 km/h. wow!
    Last edited: Jun 27, 2010
  10. Jun 27, 2010 #9
    Is your question is well written? i suspect it is not. (Please check your question again)
    What i understand is:
    Train and bird going towards each other. After 4.138 h (=720/(120+54)) train and bird will meet. So bird will cross 496.552 km (=120 km/h X 4.138 h). So for the bird to go back to starting position it will have to travel another 496.552 km.
    That means the total distance traveled by bird will be 2 X 496.552 = 993.103 km.
  11. Jun 27, 2010 #10
    I understand that bird flying toward the train, then back to "wall" then again fly to train and back to the train and back and again and again until the train reaches "the wall".

    If I'm right - total distance is 1600km and distance toward starting point is 800km.
    Last edited: Jun 27, 2010
  12. Jun 27, 2010 #11
    I dont know! In his question he did not tell anything about going back again and again..From his question i understand that the bird travel from starting position to train and then back to starting position. I gues we have to wait for his reply.
  13. Jun 27, 2010 #12
    You are right. So I understand sentence "distance towards the wall will the bird cover before the train reaches it". It make no sense (for me) if the bird flying only once. There is no matter how much time spent the bird on ending point before train reaches it.

    If you are right, chaoseverlasting just have two homework in one :smile:

  14. Jun 27, 2010 #13
    Hello Bartek,
    you notice the original question carefully. Moreover, in return journey for the bird, no need to consider the train's velocity. That why i asked him to write the question properly (scroll above). So I believe your solutions should be correct. I think he may have even more than two homework in one.
  15. Jun 27, 2010 #14
    Lets wait what chaoseverlasting say :smile:. He wrote right answer:
    I think it is from book.

  16. Jun 27, 2010 #15
    I agree with you. I thought the bird started from the wall, flew to the train, and then towards the wall and beyond it. The task is to find the distance passed from the wall and beyond (I got 607 km). But I think Bartek has understood the question correctly (his solution is also 800 km, just as in the book).

    But I think all of us agree that it is a very impressive bird flying 120 km! :smile:
  17. Jun 27, 2010 #16
    Is this speed is really high for a bird ?..Anyway this bird is amazing.
  18. Jun 27, 2010 #17
    Thank you bartek! It is from a book. I've been solving similar questions, and missed the point. A result of less thinking and blind application. :) Kudos!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook