Bobzilla wrote:trucke wrote: Which direction was the car going 25 mph headed?South, towards Albequerque. But in reality they were lost and someplace in Alabama.
I'm lost in Alabama and I'm having flashbacks from engineering school. 
I don't understand this discussion.
Someone help me!
MattGent wrote:jsquared wrote: Elastic vs inelastic collisions. In a theoretical situation where the cars are inelastic, two cars hitting at 18mph is equivalent to car hitting wall at 36mph.How is momentum conserved in an inelastic collision against a fixed object?
I didn't say momentum. On purpose, since my kinematics course was about 15 years ago and my various career detours have resulted in my not using that stuff in a while 
I may have been meaning force, but I forget 
Energy is what's conserved. Always, and forever. Energy, energy, energy.
jsquared wrote:MattGent wrote:I didn't say momentum. On purpose, since my kinematics course was about 15 years ago and my various career detours have resulted in my not using that stuff in a whilejsquared wrote: Elastic vs inelastic collisions. In a theoretical situation where the cars are inelastic, two cars hitting at 18mph is equivalent to car hitting wall at 36mph.How is momentum conserved in an inelastic collision against a fixed object?
In an inelastic collision as you referred to, momentum is conserved. That's the only way you could come up with the 36mph figure. Conservation of momentum is the right parameter, but can't be applied in the brick wall thought experiment with an inelastic assumption. The car has momentum going in, but nothing has momentum going out. In the two car version, the momentum terms are equal and opposite; ie zero.
Summary: two cars hitting at 18mph is not equivalent to one hitting a brick wall at 36, whether elastic or inelastic assumptions are made.
Well, crap.
I guess "use it or lose it" rings true here
Also, all of this only applies to cars of equal weight AND construction. Otherwise other factors come into play. This is part of why those "lets crash a new car into an old one" videos look much more dramatic compared to historical footage of similar cars in the same situation. The old one is now doing all the crinkling(energy absorbtion), as the new one is built much stiffer.
A fun one is what happens when you crash a Smart, which tests very well, into a bigger car. https://www.youtube.com/watch?v=he6TL15pJtw
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