[Someone]

That impact wasn't as large as your remark make it appear to be. The twin towers didn't collapse on impact, they collapsed from the energy released by burning the fuel in the planes and the burnable stuff present in the planes and the towers themselves (in what proportions, I don't dare take a stab at)

On the other hand, as another poster already said, you should compute energy. That goes with velocity squared, so it will be way higher these projectile, compared with a jumbo jet.

Edit: as another poster said, you also should have said miles/hour. That gives you http://www.wolframalpha.com/input/?i=25+pounds%2F2+*+((4500+...), apparently about what a Samsung S3 uses in a year (what am I doing wrong? http://www.wolframalpha.com/input/?i=25+pounds%2F2+*+((4500+... gives me 6.374 kWh. Seems a tiny amount)

[matt-attack]

To reiterate, it's worth reviewing the collisions. You'll see the buildings don't make the slightest movement upon impact. No discernible affect.