# Homework Help: Car crash physics

1. Nov 18, 2012

### zachk19

I need to know the velocities before and after the collision and the engery lost. I have some of it figured out I am just stuck and could use some help. Here is the entire problem.

The situation: a patron has just entered his car after lunch in the parking lot of the Cliff-Side Café. Suddenly, another car comes careening from the road across the street. The second (incoming) car smashes into the parked car, sending it (and its occupant) flying off the cliff. The second car itself comes to a stop, and its occupant, though shaken, is essentially unhurt. The driver of the incoming car claims his brakes failed coming down a hill, so he was unable to avoid the collision (his brakes are so damaged from the collision that they give us no information). To complicate matters, the two drivers were rivals for the same lady’s romantic affections. Given the physical parameters of the situation, does the alibi of the incoming driver hold up?
The local District Attorney is prosecuting the case as a homicide. The D.A. wants you to testify as an expert witness (think “CSI”). You are not to render a “guilty” or “not guilty” verdict, though; rather, you are to comment on the physics of the case.
• Height of the cliff: 40 meters
• Horizontal distance (from base of cliff) of crashed car: 30 meters
• Mass of each car: 1000 kg
• Coefficient of static friction for parked car: 1.0
• Coefficient of kinetic (sliding) friction for parked car: 0.50
• Distance cars slide (during collision) before parked car falls off cliff: 2.0 meters
• Coefficient of rolling friction for incoming car: 0.020
• Diameter of tires: 0.60 meters
• Mass of individual tire: 20 kg
• Coefficient of elasticity (during collision): 0.75
• Speed limit of road: 25 mph (40 kph)
• Cross-sectional area of incoming car: 1.4 m by 1.6 m
• Coefficient of drag: 0.38
• Density of air: 1.293 kg/m3
• Distance from base of hill to collision site: 50 meters
• Angle of hill: 10 degrees
• Length of hill (along slope): 18 meters
• Initial velocity of car starting down hill: 5mph (2.24m/s)

Last edited: Nov 18, 2012
2. Nov 18, 2012

### haruspex

Why and how would a car be cleaning the hull of a ship? Oh, you mean careering!
It is a requirement on this forum that you post your own attempt at a solution, and any equations you believe to be relevant.