Explainer

# The Ferrari That Split in Half

## How do we know it was going 162 mph when it crashed?

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The case of the rare Ferrari Enzo sports car that crashed and split in two on the Pacific Coast Highway moved into the courts on Monday. Prosecutors charged the man who seems to have been at the wheel with embezzlement, grand theft, drunk driving, and weapons possession. According to the Los Angeles County sheriff’s office, the car was traveling at 162 mph at the time of the accident. How do investigators figure out the speed of an auto collision?

They look at where the car and all its pieces end up. (Bits of the Ferrari were scattered over 1,200 feet of highway.) If investigators can figure out the direction and distance each piece traveled from the site of the impact, they can infer the speed at which the car was going when it crashed.

In order to perform this kind of analysis, you first need to figure out where exactly the impact occurred. Marks and debris on the highway—like gouged pavement, broken glass, or oil stains—can give you a good idea of where the accident began and what happened to the car in its aftermath. Then, you need to find out the “coefficient of friction” of the surface on which the accident took place. This tells you how quickly a car (or a piece of car) would skid to a stop on that surface—whether it’s pavement, dirt, or grass.

Given these data, you can figure out the velocity of the car as it came out of the collision—which can then help you to figure out its velocity going in. In general, you know that both momentum and energy will be conserved over the course of the accident. For example, the momentum of the speeding Ferrari before the crash would translate into a momentum for each of its two halves, and for the utility pole that it crashed into (and knocked upside down). The same goes for energy: If you know the amount of energy it takes to snap a utility pole in two, or the amount it takes to break a Ferrari in half, you can work backward to figure out how much energy the Ferrari must have had just before the impact.