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Homework Help: Force investigation

  1. Mar 3, 2005 #1
    hello, i have a worksheet to do for physics, i am almost done, i need just this problem that i couldn t solve, i just need someone to tell me where to begin or give me some hints :)
    A police officer is invistigating a traffic accident. two cars collided at an intersection. both drivers said that the other driver was at fault, because they ran a red light. the officer examined the skid marks of both cars. Car A (mass=1234Kg) had skidmarks of 338 feet, while car B (weight=11000N) had skid marks of 82 m. the driver of car A said he was going 35 mph, the driver B said he was going 45moh. Who was lying???? (the coefficient of friction = .25 for the pavement)
    please, just tell me how to interpret this problem, i didnt get the situation . :confused:
    give me some hints :smile:
  2. jcsd
  3. Mar 3, 2005 #2
    You know the combined mass, so you know the force they exert on the ground. If you know that, you should be able to find the decceleration due to friction as the wreckage slides. If you know how far they skid, you know how much time it took, and therefore you can determine their starting velocity immediately after the collision, using a standard kinematics equation involving displacement, velocity, acceleration, and time.

    Once you know that, you can find out who had what velocities at the beginning by using a property of moving objects with known mass and velocities. Hint: it's a conserved value... :)
    Last edited: Mar 3, 2005
  4. Mar 3, 2005 #3
    ok ok, but i have a problem, what's a kinematic equation? and what's a conserved value? :uhh:
  5. Mar 3, 2005 #4
    use energy to solve this problem
  6. Mar 3, 2005 #5
    I misread the problem, sorry. listen to qwerty. You can find the work done by friction on each car, so that should help you work back to original velocities.
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