2D inelastic collision and K of CM

  • Thread starter sci-doo
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  • #1
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Homework Statement


Car 1 (1500kg) is heading north 80km/h, Car 2 (3500kg) is heading east 50km/h. They collide inelastically.
a.) Speed and direction of the wreck (car 1 + car 2)?
b.) The kinetic energy lost?
HERE I HAVE A PROBLEM:
c.) What is the kinetic energy of the two-particle systems center of mass before and after the collision?
d.) Where was the lost kinetic energy hidden before the collision?

2. The attempt at a solution
a. is just vector sum p1+p2 where I can get the velocity by dividing it with the masses (about 12m/s), I can get the angle too (34,438... deg)
b. wrecks K minus K of car 1 and car 2 = - 360532,40... J so about 360 kJ lost
c. But this one I don't understand. I'd say it stays the same, because... well, that's what I calculated (CM from the v-vectors = v of CM), yet I don't really belive it...
d. ... after all shouldn't the K be lost to deformation of bodies (+heat etc.)? Does the K of CM stay the same anyways?
 

Answers and Replies

  • #2
23
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Ok, I found something useful on the net:
http://courses.washington.edu/ph122mo/A08/MT3info.html" [Broken]
# Inelastic collisions

* momentum is still conserved
* kinetic energy of center of mass motion cannot be lost
* maximum energy loss for items sticking together after collision


And that is really all I needed to know. :approve:
 
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