My favorite exam question comes from the final exam in an introductory mechanics course: A rolling 31 ton railroad boxcar collides with a stationary flatcar. The coupling mechanism activates so the cars latch together and roll down the track attached. Of the initial kinetic energy, 38% dissipates as heat, sound, vibrations, mechanical deformation, and so forth. How much does the flatcar weigh?

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Rotational dynamics, Chemical elements, Educational aids, Educational assessment, Teaching, Editorials
1.
This question is modified from problem 47 in Chapter 10, “
Collisions
,” of
David
 
Halliday
 and
Robert
 
Resnick
,
Fundamentals of Physics
, 3rd edition (
Wiley
,
New York
,
1988
), p.
224
. That problem uses a caboose rather than a flatcar, but today's students have never seen a caboose.
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