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2D Glancing Collision Problem

  1. Feb 22, 2009 #1
    1. The problem statement, all variables and given/known data
    Car two is traveling due North and collides with Car one, which is traveling due west. Car One has a mass of 765kg and after the collision it travels at 70km/h, 57 degrees North of West. Car two has a mass of 1100kg and after the collision it travels at 41km/h, 44 degrees North of West. What is the velocity of each car right before impact?


    Car One - m1 = 765kg, v1f = 70km/h

    Car Two - m2 = 1100kg, v2f = 41km/h

    2. Relevant equations

    Pyi = Pyf
    Pxi = Pxf
    P = mv

    3. The attempt at a solution

    So, I started by constructing a vector triangle for each car, and using the sine law to determine the other sides.


    Where one side is the west (x plane/ P1) momentum and one side is the north (y plane/P2) momentum. The hypotenuse (p12) is the final momentum of car one after the collision.

    P12 = mv
    =53550 kgxkm/h

    cos = adjacent/hypotenuse
    = cos44 = P1/53550
    P1 = 38520.64...kgxkm/h

    sine = opposite/hypotenuse
    =sine44 = P2/53550
    P2 = 37198.95...kgxkm/h


    Much the same as the first triangle, except the hypotenuse of this triangle represents the final momentum of the second car.

    P12 = mv
    = (1100kg)(41km/h)

    cos57 = P1/45100
    P1 = 24563.22...kgxkm/h

    sine57 = P2/45100
    P2 = 37824.04...kgxkm/h

    Okay, up to this point, I'm fairly confident that I'm on the right track. It's right here where I'm getting confused. I know I need to add the vector components, but I'm not entirely certain how.


    Pyi = Pyf

    Py1 = P2 + P2
    =37198.95...kgxkm/h + 37824.04...kg x km/h
    = 75022.99...kgxkm/h

    Pyf = final momentum of car two
    = 45100 kgxkm/h

    ...They do not match. I have accomplished nothing.
    My teacher assures me I am using the correct formula, but clearly, I am using it incorrectly. What am I doing wrong?
  2. jcsd
  3. Feb 22, 2009 #2


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    Homework Helper

    I would rather see you resolve the final mass*velocity vectors (momentum vectors) into x and y components and then combine the vertical components of each and recognize that those components belonged to the vertically traveling car before the collision and all the horizontal momentum must have come from the horizontally traveling car.
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