**Ask a monkey a physics question thread**

[OngBonga]

The density of Tungsten is ~19.25 g/cm^3. The density of the Earth's crust is ~2.2 g/cm^3
It's going to penetrate (19.25 / 2.2)*L ~= 7.5 times it's length.

This appears to be missing a factor.

For example, according to this calculation, the rod will penetrate exactly the same depth at 10mph and MACH 10, because its velocity is not taken into account. I only see density and length being considered.

What am I missing here?

It's at least plausible that these things could be up there.

Um, really? They could afford to take these things up there? My skeptisism is evaporating.

Since when has the US military let a little funding slow them down?

I wasn't thinking of a "little" funding, I was assuming it would cost a ridiculous amount.

Your argument amounts to saying "Since they're evil, they're definitely doing every evil thing that I can imagine."

Not at all. I haven't accused the oil companies of the recent terrorist attack in Nice. All I've accused them of in this thread is stifling energy technology, which is something they would have a vested interest in doing.

[MadMojoMonkey]

This appears to be missing a factor.

For example, according to this calculation, the rod will penetrate exactly the same depth at 10mph and MACH 10, because its velocity is not taken into account. I only see density and length being considered.

What am I missing here?

I'm guessing you didn't click on the wiki link I posted. The wiki page is very short, but here's the meaty bit:

Newton's approximation for the impact depth for projectiles at high velocities is based only on momentum considerations. Nothing is said about where the impactor's kinetic energy goes, nor what happens to the momentum after the projectile is stopped.


The basic idea is simple: The impactor carries a given momentum. To stop the impactor, this momentum must be transferred onto another mass. Since the impactor's velocity is so high that cohesion within the target material can be neglected, the momentum can only be transferred to the material (mass) directly in front of the impactor, which will be pushed at the impactor's speed. If the impactor has pushed a mass equal to its own mass at this speed, its whole momentum has been transferred to the mass in front of it and the impactor will be stopped. For a cylindrical impactor, by the time it stops, it will have penetrated to a depth that is equal to its own length times its relative density with respect to the target material.

Put simply, the speed of the ground being shoved out of the projectile's way is equal to the speed of the projectile, and it cancels out.

Um, really? They could afford to take these things up there? My skeptisism is evaporating.

I wasn't thinking of a "little" funding, I was assuming it would cost a ridiculous amount.

The cost of producing the projectiles is moot. The US armed forces pay through the wazoo for the highest tech munitions that are available. Furthermore, the projectile needs no warhead, so it's significantly cheaper than one which does require a warhead.

The suggested mass of the projectiles is well within max. payload specifications of space-faring rockets.

The US space program exists and our military has private access to it. I think it's a bit naive to assume that they wouldn't be weaponizing space to some degree.

Not at all. I haven't accused the oil companies of the recent terrorist attack in Nice. All I've accused them of in this thread is stifling energy technology, which is something they would have a vested interest in doing.

Fair enough... it's not "every" evil thing you can imagine... just the ones you suppose they have a "vested interest" in.

They also have a vested interest in making boatloads of money and if they have the money and power to suppress some tech, they have the money and power to own that tech and profit off of it.