How to back calculate vehicle speed after crash?

  1. Nov 9, 2011 #1
    I am struggling on solving a car crash question. Could anyone help me out?

    Car 1 (C1) weighs 3,000kg, C2 weighs 12,00kg. C1 was at rest and hit by C2 from side. Both vehicles moved 4m in 1 second before total rest. In other words, C1 was pushed away from side for 4m and C2 traveled forward 10ft.

    Question: Assuming the fraction coefficient of the dry road is 0.7. What is the estimated speed of C2 before crash?

    I am not sure if I should solve this question by using Momentum (M)
    M(before)=(Mass of C1 x Velocity of C1) + (Mass of C2 x Velocity of C2)
    = (3,000 x 0) + (1,200 x V2)

    Upon crash, in the 1-second time, cars traveled 4m... so their speed after crash is 4m/s
    M (After)=(Mass of C1 x 4m/s) + (mass of C2 x 4m/s)
    =(3,000 x 4) + (1,200 x 4)

    Now if I assume M (before) = M (after)... then I can figure out initial speed V2
    But I did not use the friction here. Do I need any other factors for the calculation?

    Or should I approach this question from kinetic energy approach
    C2 was moving with kinetic energy: F2 = 0.5x (mass of C2) x (Velocity of C2)2
    C1 was at rest, so no kinetic energy.
    But I do not know how to proceed from here

    Any help is appreciated!
  2. jcsd
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