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27 December 2006 — Story Problem (4)

You own a box factory situated in the middle of the country.

Your delivery driver is returning from a trip. As every driver has done several times a day for twenty-one years, he idles up to the edge of the parking lot, angles across the entire road, and begins to back in.

Meanwhile, a woman in a Ford Focus comes flying around the corner (which is 500 feet down the road) and accelerates toward the the truck. The woman shows no signs of stopping. At the very last possible moment, she slams on her brakes. The tires smoke and squeal as the Ford Focus comes to rest three or four feet from the rear of the truck. After the truck has backed into the drive, the woman in the Ford Focus speeds away.

You happen to witness the whole incident because you were watching for the truck's return. You go out to measure the skidmarks. They're 45 feet, five inches long. (That's 13.84 meters to those so inclined.)

Question one. How fast was the Ford Focus traveling at the time the driver slammed on her brakes? (I own a Ford Focus, too. My owners manual indicates the car's mass is 3640lbs, or 1651kg.)

Question two. If an accident had occurred, who would be at fault? (This is in Oregon.) The truck driver followed all legally prescribed procedures except honking his horn — not that the woman could have heard it.

Question three. Why do people insist on driving so darn fast?


Comments
On 27 December 2006 (10:27 AM), Peter said:

Hi JD,

1. My back of the envelope calculations show a speed of 77.36 km.hr or 48.07 mi.hr. That's assuming the mass given, although it was probably greater due to fuel, occupant and cargo weight etc. I also used a value of 1.7 for the coefficient of sliding friction between the tires and road. Given a lower amount of friction due to rain older tires the car was probably traveling at a slower speed, but at least you have a ballpark idea.

My guess is the driver was either lost in thought or busy eating/drinking/changing the radio/talking on the phone and was surprised by the truck in the road. Still, it would have been a very stupid accident and entirely avoidable.

2. No idea what the laws in Oregon are but in Canada it would probably end up being a "no fault" kind of thing and everybody's insurance would go up next year. Don't get me started on this one.

3. Blame it on the darn video games and our collective ADD.


On 27 December 2006 (10:29 AM), Peter said:

Er. Make that km/hr and mi/hr. One. Key. Over.


On 27 December 2006 (11:24 AM), John said:

Between 30 and 35 mph according to a handful of sites that Google returned:

http://www.google.com/search?hl=en&q=how+to+determine+speed+from+skid+marks&btnG=Google+Search

John


On 28 December 2006 (09:31 AM), Lisa said:

I vote that she saw the truck but was annoyed that it was there. Rude drivers are depressing.