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2D Glancing Collision Problem

  • Thread starter cybernerd
  • Start date
1. Homework Statement
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?

So

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

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


2. Homework Equations

Pyi = Pyf
Pxi = Pxf
P = mv
SINE LAW


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.

CAR ONE TRIANGLE

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
=(765kg)(70km/h)
=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

CAR TWO TRIANGLE

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)
45100kgxkm/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.

So...

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?
 

LowlyPion

Homework Helper
3,079
4
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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