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Hard kinematic problem, need help pls

  1. May 6, 2008 #1
    hard kinematic problem, need...help...pls:)

    1. The problem statement, all variables and given/known data
    A policeman (on a motorcycle, with the engine going) is hiding behind a bush on the side of the road when a speeding truck passes with a speed of 96.5 km/h. As the truck passess he sets off with an acceleration of 1.83 m/s, which he maintains until he overtakes it. Calculate:
    a) the time he took to overtake the truck
    b) the distance he travelled in doing so
    c) the velocity, in km/h, of the motor cycle when it catches the truck.


    2. Relevant equations


    3. The attempt at a solution

    Well first i converted 96.5 hm/h into m/s which is 26.8 m/s and i know that his acceleration doesn't change, im just wondering is the policeman's final velocity the speed that the truck is travelling at which is 26.8 m/s? And is his initial velocity 0? and his acceleration is 1.83 so i thort that i cud use v=u+at to work out a)
    i think im on the wrong track though actually I pretty sure I am. Can someone pls help me and also can sum1 pls tell me sum things that i shud look out for when trying to solve these really hard straight line motion questions. Like i get the simple ones but it is just ones similiar to these that i find difficult. I appreciate all the help!! Thankyou!!:)
     
  2. jcsd
  3. May 6, 2008 #2

    Hootenanny

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    You can't assume that the final two velocities are the same, in some cases it may very well be that the two have the same final velocity, but this is not generally the case.

    Instead, what do you know about the distance between the truck and the bike at the time your trying to find?
     
  4. May 6, 2008 #3
    um i dont know a distance.. or maybe I cannot see it
     
  5. May 6, 2008 #4

    Hootenanny

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    When the bike is level with the truck, where is their separation?
     
  6. May 8, 2008 #5
    they're at the same distance?
     
  7. May 8, 2008 #6
    so we need to find the time when the distances are the same?
     
  8. May 8, 2008 #7

    Hootenanny

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    Correct :approve:
     
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