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Reply #199: Which weighs more... [View All]

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William Seger Donating Member (1000+ posts) Send PM | Profile | Ignore Wed Nov-11-09 07:00 PM
Response to Reply #190
199. Which weighs more...
... 50,000 tons of intact building or 50,000 tons of building debris? Of course they would have the same mass, so they have identical kinetic energy when moving at a given velocity.

For simple physics calculations, in the equation for force due to acceleration (or deceleration) F = ma, the mass m is generally taken as a single point-like object. In that simple formulation, it would not matter if the falling mass was intact building or debris. However, it is true that they would not really deliver quite the same impulse force, because the impact would be spread out somewhat in time rather than all at once. But Bazant does not use that simplified formulation: His calculations include a term to distribute both the velocity and the "momentum density" linearly through the compacted debris layer. That improve the precision of the differential formula he derives, but it's still important to note that Bazant's primary argument is that the kinetic energy of the falling mass was much larger than the structure could absorb.
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