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Kinematics cyclists problem

  1. Sep 30, 2011 #1
    1. The problem statement, all variables and given/known data
    Two cyclists, A and B, have displacements 0 km and 70 km, respectively. At t=0 they begin to cycle towards each other with velocities 15 km/h and 20 km/h, respectively. At the same time, a fly that was sitting on A starts flying towards B with a velocity of 30 km/h. As soon as the fly reaches B, it immediately turns around and flies towards A, and so on until A and B meet.

    (A) What will the displacement of the two cyclists and the fly be when all 3 meet?
    (B) What will be the distance traveled by the fly?


    2. Relevant equations
    Not too sure about this one... maybe d= vit + 1/2 at2


    3. The attempt at a solution
    All I know is the answer for a is 30 km/h and 60 km/h for b
     
  2. jcsd
  3. Sep 30, 2011 #2

    PeterO

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    You can work out the closing speed of the two cyclists, and thus the time that will elapse before they meet.
    Each cyclist has been doing their speed for that time, so you can work out where they will be.
    The fly has been doing its speed so you can work out the distance covered by the fly.

    Note:
    I think the word displacement in the question, and part A is supposed to be position.
    Displacement is by how much your position changes. When the problem starts, no-one has any displacement, though one cyclist is at a position 70km from the other.
     
  4. Sep 30, 2011 #3
    So you recommend I draft a rough position vs time graph?
     
  5. Oct 1, 2011 #4
    bump: still not getting this any help appreciated
     
  6. Oct 2, 2011 #5

    SammyS

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    How long does it take the cyclists to meet?
     
  7. Oct 2, 2011 #6
    I don't know I'm asking for assistance.
     
  8. Oct 2, 2011 #7

    PeterO

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    After one hour, where are these two cyclists?

    They were originally 70 km apart - how far apart are they after 1 hour?

    That will tell you their closing speed.
     
  9. Oct 2, 2011 #8
    After one hour they are 35 km apart. Thus, after two hours, they'd meet. I see... :) Now how would I use that in a formula is the question.
     
  10. Oct 2, 2011 #9

    PeterO

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    What formula did you just use then?
    If you didn't use one, then you don't need a formula now.

    If they meet after 2 hours, where is Cyclist A? Where is cyclist B, where is the fly?
     
  11. Oct 2, 2011 #10

    PeterO

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    Use this answer, and re-read post #2
     
  12. Oct 2, 2011 #11
    cyclist a is 30 m distant from start, b is 40 but meets at 30, and the fly is 60 but meets at 30.


    I have this feeling that i know what it is but answering it is confusing, like the other two have reached before cyclist a .
     
  13. Oct 2, 2011 #12

    PeterO

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    The fly will meet each of A and B frequently during the 2 hours - but Cyclist B will meet Cyclist A only the once.

    The fly travels from A - B - A - B - A - B - etc so is always between A and B [or with one of them] When A and B are together, with A, with B and between A&B are all the same place.

    Your opening statement could have been more specific.
    ie:
    A is 30km from his starting point [0] so is at 30km
    B is 40km from his starting point [70] so is at 30km
    The fly is also there.
     
  14. Oct 2, 2011 #13
    Thanks a lot, you really helped me understand this better. From where I started, I actually feel idiotic now haha.
     
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