Gravity is an important force, bro. You may remember from your high school math or science class that everything has a terminal velocity. This concept is 99% of the time thought of something falling through air, a gas. Or more specifically, a mixture of gases. If you threw a pillow, a dart, and yourself out of an airplane, you would reach the earth at different times. With different consequences. The maximum speed you would reach from gravity alone (as opposed to being shot out of a canon, towards the ground) is your terminal velocity. Or, more eloquently put:
"A free-falling object achieves its terminal velocity when the downward force of gravity (Fg) equals the upward force of drag (Fd). This causes the net force on the object to be zero, resulting in an acceleration of zero.[1]"-
"Higher speeds can be attained if the skydiver pulls in his or her limbs (see also freeflying). In this case, the terminal velocity increases to about 320 km/h (200 mph or 90 m/s),[2] which is almost the terminal velocity of the Peregrine Falcon diving down on its prey.; The same terminal velocity is reached for a typical .30-06 bullet travelling in the downward vertical direction — when it is returning to earth having been fired upwards, or perhaps just dropped from a tower — according to a 1920 U.S. Army Ordnance study.[3]"
"The reason an object reaches a terminal velocity is that the drag force resisting motion is approximately proportional to the square of its speed. At low speeds, the drag is much less than the gravitational force and so the object accelerates. As it accelerates, the drag increases, until it equals the weight. Drag also depends on the projected area. This is why objects with a large projected area relative to mass, such as parachutes, have a lower terminal velocity than objects with a small projected area relative to mass, such as bullets."
-Wikipedia
Let's Fall
In the beginning of that Wikipedia article, it mentions terminal velocity is a property of fluid dynamics. Not only does something falling through air have a terminal velocity, the same is true for liquid (i.e. water). A sinking ship, a lead weight, or a dolphin corpse. OK, hopefully you are intrigued with all the talk about falling objects, but probably don't know where this is going. Well, recently there was a study in Mexico and Cuba that showed objects travelling threw a granular medium (i.e. sand, ball pit, polystyrene beads) could reach a terminal velocity. This is important because it has not even been publicly theorized before. If anyone has watched Through the Wormhole with Morgan Freeman, that is impressive in it's own right.
The link is below for the Pop-Sci article, and it really caught my attention. The researchers dropped (or more accurately, shot) different weighted ping-pong balls into the polystyrene (i.e. foam) beads and observed them to reach a terminal velocity. Imagine jumping into a ball pit and falling forever, or dropping something on a sandy beach and have it sink beyond reach.
Heavy Metal
In that test, the researchers modified the density of the balls by filling them with steel particles/ dust. Their heaviest ping pong ball, at 182 grams, was still less than 1/2 a pound. By doing more math then I should be logically be doing in my normal life, I calculated that the ball should be closer to 9+ ounces if filled with solid steel or iron. This means that the some density has been lost due to the granular nature of their product, and probably completely topped off with the steel particles.
Why is this important? In the physorg.com original article, they mention a ping pong ball would have to weigh approx. 31 pounds to travel endlessly through sand. Well, that stat sparked my interest for this entire blog post. Inside my head " Wow, 31 lbs. How much steel would have to be in it? I don't think that would work. They just did the test with steel. Steel is really heavy. Is there anything heavier than steel? google.com"
At that point, I came head on with my wrong assumptions:
#1 a 31 lb. ping pong ball is possible
#2 Steel is really heavy
This search lead me to a few very interesting websites, and some strong realizations. Refer back to my 1st sentence about a Tungsten ping pong ball. I calculated this to weigh a little more than 1.42 lbs, or about 23 ounces. That is about 2-1/2 times the weight, or density of steel! Tungsten is pretty much the heaviest metal we have hear on earth that is safe to handle. There are a few substances that are only marginally heavier, such as Uranium. So, it's safe to say the 31 lb. ping pong ball isn't happening anytime soon.
On a side note, a Tungsten ping pong ball would be fun to toss to your friends if you like giving them stress fractures or hideous bruises. Also, I would like to start a gym called Tungsten where all the weights are: you guessed it, Tungsten. I think it would be cool to have 45 lb. plates about 25% smaller than other gym's 25 pounder's. I'm sure the cost would be astronomical.
Depleted Uranium
I recently did some light reading on Depleted Uranium (DU) shells, and it fit right into this whole conversation. The difference between DU Uranium and Enriched Uranium- EU can be used for nuclear fission. I found a great article on the Santa Clara University website, linked below. The concept behind a DU shell is the same reason a "bullet proof" vest does not stop a sharp knife. You want a concentrated force to penetrate the armor, instead of being dispersed. For comparison, DU is 68.4% more dense than lead. These heavy shells are very dangerous and strongly linked to radiation poisoning ("Gulf War Syndrome"). DU shells also offer some different benefits compared to other advanced ammunition, as stated in the article. Research into using these DU and Tungsten slugs started in the 1970's. DU is also used for advanced armor. It's amazing how the military will gravitate towards a material because of it's low cost, despite being potentially poisonous to your own troops. And it's density.
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| 120 mm Tank Ammunition |
Phys Org Article (original)
Terminal Velocity Through granular medium (Popular Science)
Volume of a Ping Pong Ball
DU Ammunition- SCU
