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Q & A: forensic physics

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Most recent answer: 10/29/2010
Q:
My wife was involved in a (car v m/cycle) accident a few weeks back. The m/cyclist was speeding and drove into her, writing off her car. Is it possible to calculate his approximate speed just before he hit the car, from the distance his crash helmet travelled after it came off of his head and/or the distance he travelled after hitting the car. His vehicle obviously stopped dead after hitting the car. He had not done up the chinstrap to the helmet before the accident. I will in no way hold you accountable for any legal issues that could arise from this you answering this question! The distance he travelled after hitting the vehicle was approx 25ft and he weighs approx 170 pounds. The helmet travelled approx 80-100 feet and it weighs approx 3.5lbs. His head height was approx 4ft 6in above the ground at the time of the accident. Good driving conditions, tarmac road surface, Car was a Renault Megane Scenic. Bike was a Kawasaki single cylinder 125cc model. I saw no skid marks on the road from the bike. I can think of no other information you may need, however, If you need any further information on this please contact me as best you can.
- John Bates age 55
Leeds, West Yorks, U. K.
A:
This is too hard for us. However, if you search for some physics expert witnesses, it sounds pretty close to a standard sort of problem on which they frequently are called to testify.  Perhaps experiments could be run to test the behavior of the same type of helmet at the same place, giving both an estimate and some error bars. I doubt if anyone will volunteer for experiments to test the distance vs. speed of the motorcyclist,

Mike W.

(published on 10/29/2010)

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