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You are just starting a new job as investigator for the carcollision investigation. You assist your superior in order todetermine, as accurately as possible, the circumstances surroundingcollisions (initial velocity of the vehicles involved, directionvehicles were heading, etc.) in order to determine whether thedrivers at fault under the Criminal Code (dangerous driving causingdeath or other charges).

The weather is great and you arrive at the station as your bosstells you, “Hey, Daniel, hurry over to Jalan BBN 1/A. There hasbeen an accident involving two cars. There are people injured.” Yourush to the scene of the accident immediately. Your job is to takephotos of the vehicles as they are positioned, and take a certainnumber of measurements, including the length of the skid marks andlayout of the debris.

Your findings are as follows:
• Numerous debris (broken glass, plastic, etc.) were found at adistance of 12 m from the cars;
• The two cars stuck together (red car is heading to the black car)and there are skid marks over 12 m;
• There are also skid marks over a distance of 30 m before thedebris;
• The posted speed limit on this street is 70 km/h;
• Other information: m1(red car) = 2500 kg, m2(black car) = 1100kg.

Return to the station, your superior yells out, “Hey, Daniel, makeyourself useful! Bring the friction block and spring scale, andtake the measurements needed to calculate the pavement’scoefficient of friction.” Confused, but very eager, you take theheavy iron friction block, covered with tire treads, and the springscale.

Your superior returns to you and says, “The two drivers aren’tsaying much. The one in the black car was sent to the hospital. Thedriver in the red car is still discussing what happened, butnothing is clear!” Since you are always willing to help, youanswer, “How can I help you, Sir?” Your superior’s face lights upwhen he hears your proposition, “Well, it’s that…, you see, we mustbe sure, beyond any doubt, that dangerous driving was involved… thejudge needs evidence. We must gather all the necessary evidence.All I’m sure of is that, based on the skid marks, the black car wasat the stop when the collision occurred. This a perfect job for anintern! Get to work! I want a full report as soon as possible”.Knowing that this information will decide whether the driver willbe charged under Criminal Code, you take your role very seriouslyand immediately get to work.

Considering that the accident occurred in a 70 km/h zone, what doyou suggest to your superior? How would you clearly explain to yoursuperior about the method used to determine the speed of the driverat fault?

Learning outcomes:

Successful students will be able to:
1. Calculate the velocities of two vehicles before and after impactusing physics principles, such as forces, motion, mechanicalenergy, and conservation of momentum.
2. Explain how frictional forces related to varying surfaces affectthe motion of an object.