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Car and the fly!

  1. Jul 27, 2010 #1
    1. The problem statement, all variables and given/known data

    A car moving at constant speed of 40 km/h in a straight road which heads towards a wall makes a 900 (90 degree) turn by the side of the wall. A fly flying at constant speed 100km/h starts from wall till car and when it reaches the car turns back and flies towards wall at same speed (100 km/h). This procedure starts when the car is 20 km away from the wall. The fly continues its journey till the car takes the 90 - degree turn.
    So how many trips has the fly made between the car and wall?

    2. Relevant equations
    distance = speed * time

    3. The attempt at a solution

    [PLAIN]http://img294.imageshack.us/img294/2922/suma.png [Broken]

    A is the initial position of car. B is the position where the fly first meets the car. O is the wall.
    So let x = 20km be distance from car to wall.
    Let t be time taken for car to reach from A to B

    AB = 40t
    OB = 100t
    x = AB + OB
    t = x/140

    OB = \frac{5x}{7}

    This is all that I got...
    When I checked answers it says infinite....
    I just dont get it..
    Please Help!!!
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Jul 27, 2010 #2
    Well, the fly's velocity is initially opposite that of the car, so you've got to make one of them negative. When the fly goes back toward the wall then both velocities are the same sign. That will change your expression a bit. Sounds like you're supposed to get a diverging geometric series, if the answer is infinite.
  4. Jul 27, 2010 #3
    I didn't quite get the:
    x = x/140
    and the
    OB = 5x/7

    But the reason the fly made infinite trips, i guess, is that the closer the car gets to the wall, the quicker the fly makes a trip. And finally when the car reaches the wall, the distance of the fly's trip is 0, therefore at that moment it will make infinite number of trips.

    Hope I didn't just lie to you :)
  5. Jul 27, 2010 #4


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    Do yo need to find the total distance the fly flies or are you looking for the number of back and forth trips?
  6. Jul 27, 2010 #5
    Oh sorry about that
    its like this
    t = x/140

    substituting t
    OB = 100 t = 100x/140 = 5x/7

    But .... I feel this may be wrong. (no offense)

    I need to find the number of back and forth trips.
  7. Jul 27, 2010 #6
    you are considering velocity.... instead I have taken it in terms of speed according to the question(hope I did the right thing)

    And about the diverging geometric series......... could you please tell me what it is (I just know geometric progression)
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