# Car Crash Web Quest

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Car Crash Web Quest Ray Wagner, Pat Suter, Mike LiPani

12/21/11 Physics R (3+4)

A traffic accident has just occurred on Millway Street. A 3000 kg Cadillac Escalade SUV has just rear-ended a 2000 kg Subaru Outback at a stop sign. We are trying to determine if the vehicle exceeded the speed limit of 35 km/hr before the time of impact. The results of this investigation will determine the punishment of the SUV driver.

We examined the crash site and found 24 meter skid marks from the wagon and 2 meter skid marks from the SUV. The drivers evidently slammed on their brakes after impact. Furthermore, we examined each vehicles braking capabilities and discovered that with the brakes locked, the SUV will accelerate at -2/ms^2 and the wagon will accelerate at -3 m/s^2, meaning the vehicles will slow down at the said rate when the driver floors the brakes. Since we know the acceleration, initial velocity of the Subaru, breaking distance for both vehicles and the final velocity of the SUV, we can use out kinematic equation (vf^2=vi^2+2ad) to individually calculate the final velocity of the Subaru and the initial velocity of the SUV before impact.

Our collision experts have determined that the SUV reached a speed of 2.83 m/s immediately following the crash and the wagon reached a speed of 12 m/s. In order to determine if the SUV was speeding before the collision, we had to find the momentum of both vehicles. We multiplied both vehicle’s masses and change in velocities to get the total momentum. We found the momentum of the SUV right after the crash to be 8490 newtons/s and the momentum of the wagon to be 24000 n/s. Since the law of Conservation of Momentum states that the momentum of an explosion is the same before and after, we could combine the each momentum to get a total momentum of 32490 n/s without any extra work. This probably means nothing to you at this point but it’s essential to the last part of our investigation. Momentum equals mass times velocity so we did some simple algebra and divided the total momentum into the mass of the SUV and found its velocity of 10.83 m/s. As you know, speed limits are not measured in meters per second, so we converted meters per second to kilometers per hour. The SUV was traveling 38.953 km/hr, 3.953 km/hr too fast. So the driver was in fact speeding before impact.

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