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Car Crash Problem

  1. Aug 18, 2008 #1
    1. The problem statement, all variables and given/known data

    There are two cars and the situation is located at a crossroad. car 1 is travelling south and car 2 is travelling east. For some strange reason car 2 has slammed the brakes and stopped right in the middle of the intersection, causing car 1 to 't-bone' crash into car 2. the mass of the cars are given:

    mass(car1) = 1875 kg
    mass(car2) = 1051 kg

    the distance of car1's skid marks are = 16m

    The speed limit on the crossroad is 50 km/h, the question of this problem is, was the driver of car1 speeding?

    2. Relevant equations

    s = ut + 1/2at^2
    F(friction) = [tex]\mu[/tex]mg
    P=mv ?

    3. The attempt at a solution

    i think what the question is asking for is 'u', the speed car 1 was doing before he applied the brakes.
    Therefore i used s = ut + 1/2at^2
    however i am not given acceleration or time, 's' is the displacement of when car1 applied the brakes to when he hit car2, therefore 's' is the distance of the skidmarks ( assuming the car make skidmarks as soon as the driver applied the brakes :P) s = 16m

    Using the Friction equation, F(friction) = [tex]\mu[/tex]mg i found the acceleration

    F = [tex]\mu[/tex]mg
    ma = [tex]\mu[/tex]mg
    a = [tex]\mu[/tex]g
    a = 0.8 x 9.8
    = 7.84 m s-2

    so all i need to do now is find the time of the braking , but i have tried everything and i cannot find anyway of doing it......i feel like im at a dead end, what should i do?
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Aug 18, 2008 #2


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    Homework Helper

    Welcome to PF!

    Hi calcite! Welcome to PF! :smile:

    Hint: use the ordinary collision equations, and then use work-energy (force x distance = energy lost). :smile:
  4. Aug 18, 2008 #3
    Go to QUT much?
  5. Aug 18, 2008 #4
    are you saying to use conservation of momentum???

    its inelastic so....

    mass(1) x velocity (1) + mass(2) x velocity(2) = (mass(1) + mass(2)) velocity (f)

    i don't understand what we use for the velocities.......
  6. Aug 18, 2008 #5
    lol yes, who are you?
  7. Aug 18, 2008 #6


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    Homework Helper

    Hi calcite! :smile:

    (I'm assuming car 1 and car 2 have the same velocities after the collision:)

    You know the friction force and the distance, so you can work out the energy of the cars, and from that you get the velocities v1f = v2f immediately after the collision.

    And you know v2i = 0.

    So your mometum equation gives you v1i. :smile:
  8. Aug 18, 2008 #7
    Haha, it's a secret :)
    I'm here trying to find a means for one of those quiz questions, but I couldn't help but click on your "Car Crash" topic.
  9. Jun 26, 2010 #8
    Can you be of any help, I need a simple (if that is possible) equation to help me determine the weight increase of a 12 stone occupant of a car doing 30mph coming to a dead stop?
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