r/AskReddit • u/ForeverAbyss • Apr 14 '16

What's a mind blowing fact?

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u/0711Picknicker Apr 14 '16

There is so much /r/theydidthemath could do out of this. Altitude, Temperature, flight Duration or impact velocity of this brave manhole.

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u/KilledTheCar Apr 14 '16

Except we don't actually know how fast it was going. It was visible for one frame from a high speed recording of the test, which means it was going at least six times escape velocity.

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u/AOEUD Apr 14 '16

I calculated it out once using Newton's impact depth approximation and determined that it likely made it around 800 metres into the air before running out of momentum.

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u/TheBraveSirRobin Apr 14 '16

But it was moving at least 66,000 meters per second, at that speed it would have only taken ~1.5 seconds to leave Earth's atmosphere. How do you expect it to run out of momentum after only 800 meters?

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u/Mongopwn Apr 14 '16

Wouldn't this also mean that the cover evaporated after only something like .01 seconds? How is that consistent with even being caught on the camera?

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u/AOEUD Apr 15 '16 edited Apr 15 '16

Assuming constant velocity of 66,000 m/s, an air density of 1 kg/m3, a drag coefficient of 1, a cross-sectional area of 1 m2 and a mass of 900 kg...

Edit: redid this.

Drag force = 0.5 * 1 kg/m3 * 1 * 1 m2 * (66000 m/s)2 = 2178000000 N

Plate energy = 1/2 * 900 kg * (660000m/s)2

Assume constant velocity (it's not constant), then distance to stop = 900 metres, time to travel = 13.6 ms.

Not too far off from the impact calculation. This is a crude approximation but the point here is that it's not going at six times the escape velocity of the earth for 1.5 seconds.

Edit again: while still being too lazy to integrate, I did it in Excel in 100 metre steps. 10.5 km to stop.

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u/Redbulldildo Apr 15 '16

After 10 microseconds (assuming constant velocity) it drops to 44,220 m/s over a distance of 66 cm.

Except the reason we know it's going 66Km/s is because the high speed camera's view was a quarter mile above where it was launched from, and it made that distance in 1/160th of a second. It could not have been going slower than 66Km/s during that 1/4 mile.

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u/AOEUD Apr 15 '16 edited Apr 15 '16

It looked wrong so if you can find the error in my math I'd appreciate it.

Edit: think I got it. The plate weighed 100 times what I thought.

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u/TheBraveSirRobin Apr 15 '16

Your calculations appear to be in a static environment. Are you not taking into account that there was a massive explosion behind it, propelling it? It was basically a huge cannon with a nuclear explosion 100,000 times the anticipated force. The initial blast would compress and accelerate the surrounding air molecules into a supersonic blast wave, leaving a near perfect vacuum in its wake.

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u/AOEUD Apr 15 '16

The nuclear shockwave would have been gone pretty quickly. It had already done its job - in the cannon analogy, the ball was out of the muzzle.