Sunday, November 13, 2011

Crash Test Data

We've been spending the last couple days in physics analyzing the following video:

We used Tracker Video Analysis to analyze the force the dummy felt without a seatbelt, and compare that to the force the dummy felt with a seatbelt and airbag combination. Since we have been studying momentum, I was hoping that we could find that though the dummy experiences roughly the same impulse (change in momentum) either with or without a seatbelt, the dummy experiences greater force without a seatbelt due to the impulse occuring in a shorter interval of time.  The formula for impulse is after all Δp = FΔt which yields F = Δp/Δt which is bigger when Δt is smaller.  Initial findings from the class have not been conclusive whether Δt is indeed smaller, but since the students are still writing their labs on the subject, I won't elaborate here.

I will however post the graphs I found.
First the boring data -- the car - a position versus time graph (inches and seconds are units)
 and a velocity versus time graph (in/s and seconds are units).

Now for the dummy without a seatbelt - a position versus time graph (inches and seconds)
 and a velocity versus time graph:

Finally the dummy with the seatbelt -- looking at just the initial collision (inches and seconds)
 And velocity versus time (in/sec and sec)

Since the close up view of the dummy did not show the whole picture, I did another track of the whole dummy with the seatbelt on, this shows the initial impact with the airbag, but then the subsequent hitting against the chair and finally coming to a stop. Again, units are inches and seconds
 and for the velocity graph: in/sec and seconds.

If the Δt does not prove conclusively that Force is diminished, then I may need to give a quick primer on pressure -- as I'm sure the pressure on the dummies forehead in the crash without a seatbelt (notice the glass shattering!!) is much higher than the pressure the dummy felt smothering his whole face (notice the paint left behind) on the airbag.  Otherwise my students might wrongly conclude that wearing their safety belt is worse than riding without.

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