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Momentum - Car Crash

  1. Oct 25, 2006 #1
    A police officer is investigating the scene of an accident where two cars collided at an intersection. One car with a mass of 1135 kg moving west had collided with a 1300 kg car moving north. The two cars, stuck together, skid at an angle of 30° north of west for a distance of 25 m. The coefficient of kinetic friction between the tires and the road is 0.80.

    What was the speed of the lighter car?
    ______ km/h
    What was the speed of the heavier car?
    ______ km/h

    I know this is a perfectly inelastic collistion and that Vf will be equal.
    so x-dir m1vi1 = (m1 + m2) Vf cos 30
    y-dir m2vi2 = (m1 + m2) Vf sin 30
    so i can't solve too many unknowns.
    How do i use the 25m and kinetic friction??
  2. jcsd
  3. Oct 25, 2006 #2

    Doc Al

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    Staff: Mentor

    Use this info to find the acceleration during the skid and then the speed of the cars immediately after the collision. Hint: What's the force of kinetic friction?
  4. Oct 25, 2006 #3
    F = .8 m g
    F = .8 * 2435 * 9.8
    F = 19090.4 N

    a = F/m
    a = 19090.4 / 2435
    a = 7.84 m/s

    I think this is right but not sure after.

    Vf^2 - Vi^2 = 2ax
    setting Vf to 0 I get a Vi = 19.8
    and i know this Velocity is too slow???
  5. Oct 25, 2006 #4

    Doc Al

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    Staff: Mentor

    All good. (Acceleration has units of m/s^2, not m/s.)

    What makes you think this is too slow? (Convert to miles/hour to compare with typical speeds.)
  6. Oct 25, 2006 #5
    The last part of the question states The speed limit for each car was 70 km/h. And I know that only the lighter car was speeding.

    So V1i > 70 km/s

    When i solve for my initial velocities using 19.8 and this formula
    V1i = ((m1 + m2) / m1) Vf cos 30 putting in 19.8 i get
    V1i = 36.8 km/s

    Which is considerably lower that 70 km/s

    So either my formula or my Vf is wrong but after many hours I still can't find where i'm wrong. Any other help??? Thanks so far.
  7. Oct 25, 2006 #6

    Doc Al

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    Staff: Mentor

    check your units!

    The speed limit is 70 km/hr, not 70 km/s. :wink:

    Also, standard units will give the speed in m/s, not km/s.
  8. Oct 25, 2006 #7
    Wow don't know how i missed that but thanks alot!!!
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