Question

You are just starting a new job as investigator for the car collision investigation. You assist your superior in order to determine, as accurately as possible, the circumstances surrounding collisions (initial velocity of the vehicles involved, direction vehicles were heading, etc.) in order to determine whether the drivers at fault under the Criminal Code (dangerous driving causing death or other charges).

The weather is great and you arrive at the station as your boss tells you, “Hey, Daniel, hurry over to Jalan BBN 1/A. There has been an accident involving two cars. There are people injured.” You rush to the scene of the accident immediately. Your job is to take photos of the vehicles as they are positioned, and take a certain number of measurements, including the length of the skid marks and layout of the debris.

Your findings are as follows:
• Numerous debris (broken glass, plastic, etc.) were found at a distance 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 the debris;
• The posted speed limit on this street is 70 km/h;
• Other information: m1(red car) = 2500 kg, m2(black car) = 1100 kg.

Return to the station, your superior yells out, “Hey, Daniel, make yourself useful! Bring the friction block and spring scale, and take the measurements needed to calculate the pavement’s coefficient of friction.” Confused, but very eager, you take the heavy iron friction block, covered with tire treads, and the spring scale.

Your superior returns to you and says, “The two drivers aren’t saying much. The one in the black car was sent to the hospital. The driver in the red car is still discussing what happened, but nothing is clear!” Since you are always willing to help, you answer, “How can I help you, Sir?” Your superior’s face lights up when he hears your proposition, “Well, it’s that…, you see, we must be sure, beyond any doubt, that dangerous driving was involved… the judge needs evidence. We must gather all the necessary evidence. All I’m sure of is that, based on the skid marks, the black car was at the stop when the collision occurred. This a perfect job for an intern! Get to work! I want a full report as soon as possible”. Knowing that this information will decide whether the driver will be charged under Criminal Code, you take your role very seriously and immediately get to work.

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

Learning outcomes:

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

Answer 1