[zaken]

A 25-pound projectile traveling at 4500 miles per hour carries as much momentum as a jumbo jet at cruising speed: http://www.wolframalpha.com/input/?i=25+pounds+*+4500+miles+...

We know the impact a hijacked jumbo jet can have on buildings...now imagine being able to sustain that level of destruction at a rate of 10 times/minute with hardly any power or material consumption.

[datadata]

Your calculation is wrong. You are using 4500 miles/second instead of 4500 mile/hour as the velocity.

Additionally, small really fast projectiles can more easily pack a lot of kinetic energy relative to a larger, more slowly moving projectile because kinetic energy is proportional to velocity squared. It is not so easy to pack a lot of momentum in a small really fast projectile because momentum is just directly proportional to velocity.

[dredmorbius]

The impact of a jumbo jet on a building is relatively modest. Not good to those on the directly affected floors, but even high-speed direct impacts are sustainable.

It's the jet fuel that melts steel beams.

Kinetic energy = 0.5 * m * v^2

The energy density of kerosene (jet fuel) is about 33 MJ/l

We can compare the impacts of American Airlines Flight 11, a Boeing 767-233ER weighing about 180 tonnes, with about 38,000 l of fuel aboard, with the energy capacity of that fuel, comparing both to barrels of oil equivalent, using GNU Units:

You have: 0.5 * (180 tonnes) * (748 kph)^2 You want: barreloil * 0.63494668 / 1.5749354

And for the fuel (I'm assuming 1kg/l for kerosene, this is slightly high, but the fuel quantity isn't precisely known. Rough accuracy is sufficient.)

You have: 0.907 * 38 tonoil You want: barreloil * 235.81404 / 0.0042406296

The energy of the plane's fuel is roughly 370x that of its kinetic impact.

[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.

[bunderbunder]

It's not just how much momentum is in the projectile, it's also how much energy is transferred to the target. A jumbo jet at cruising speed is likely to transfer almost all its energy into a large portion of virtually anything it hits. A small projectile traveling at 4500mph is likely to either transfer a lot of energy into a small area, or punch a hole straight through and keep on going until it hits something else.

Which isn't to say that it's not useful, but it probably isn't useful for the same applications. Something like this wouldn't be my top choice for large-scale destruction such as knocking down buildings. I sure as hell wouldn't want to be in a bunker or tank that has one of these things bouncing around inside, though.

[malisper]

You typed miles per second and not miles per hour.

[stavrianos]

Well, I mean, you have to consume enough power to get a jumbo jet up to cruising speed.

[Bromskloss]

I was under the impression that energy was a better indicator of damage, or is that more for small stuff like hand guns?

[lobster_johnson]

The destructive power of this gun comes from the energy of its momentum (mass x velocity) -- all the energy expended to accelerate it needs to go somewhere when it reaches its target. Same principle as a handgun. The difference is in how the projectile is accelerated.