This point has probably been made during this thread, but let me toss it
in again:
The question "Why does 1/2 the kinetic energy 'disappear' in a
completely inelastic collision?" is frame dependent, along with the
assignment of a particular value for the kinetic energy of the system.
Viewed from the center of mass, all of the kinetic energy vanishes.
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Today is the prerequisite
for the rest of your life.
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Dr. George Spagna
Chair, Physics Department
Randolph-Macon College
P.O. Box 5005
Ashland, VA 23005-5505