Why did Fat Man have dark paint markings?

You asked a terrific question about the black paint on Fat Man's protruding parts. Fat Man was delivered to Tinian Island in parts and assembled on the island. The implosion design had been tested in New Mexico, but had never been exposed to the kind of humidity it experienced in the Pacific. After assembly, but before loading it onto the bomber Bock's Car,  all of Fat man's seams were sprayed with a sealer. I think it was asphaltum or something similar. Many of the protruding parts were bolted on, so their seams were included in the treatment.

Are the Little Boy and Fat Man models at the BSM full scale?

Our replicas are full scale. Actually around 2005 ago we replaced our original models with two even better replicas that are more accurate in representing the appearances of the two weapons.  The two new versions, constructed of steel and fiber glass are actually ballasted with concrete to represent the actual weights of the original weapons. Not that we ever heft them.

Comparing Hardness of Rocks

We were asked how students can compare the hardness of different rocks, especially scoria and volcanic tuff.

To compare two different materials’ hardnesses, one approach is to try to scratch one with the other. We suspect, although we haven’t tried, that you will find the scoria scratches the tuff and not vice versa. This method is used by gemologists, who know that a rock that can be scratched by a common steel nail will be too soft to take a high polish. Steel is just about the perfect hardness to make this determination. Agate is harder than steel and polishes to a high luster, limestone is softer and remains dull no matter how long it is polished. A diamond should be able to scratch almost anything, and talc shouldn't scratch anything.

Ultimate Source Of Nuclear Energy

We have been asked during our energy program how nuclear energy fits into the mix when all of our other forms of energy derive ultimately from the sun.

The tie-in is actually pretty interesting. Most of the energy we use comes directly or indirectly from the sun, which is “burning” hydrogen created during the Big bang. Stars fuse hydrogen atoms into heavier elements only up to iron. The planets, and all the elements heavier than iron (further down the periodic table) are composed of stuff created in supernovae, dying exploding stars, that preceded the sun. Only a supernova is energetic enough to create the exotic and unstable elements up to uranium. So nuclear reactors, which run primarily on uranium, ultimately derive their energy also from stars, although not our sun.

Cross Puzzle with 4 pieces

You asked if the puzzle in TechLab with four pieces actually reassembles into one large square.

 Two small squares are easy.

The hint posted alongside this puzzle is that the square has twice the area of the cross. The only way this can be, using the same four pieces, is if the square has a hole in it with the same area as the cross. We will be the first to admit this is a sneaky solution.

In this case, the hole is actually the same shape as the cross. The drawing shows the cross pattern with one piece rotated into position as a corner of the square pattern.

Teenage Brain Question

As a middle school teacher, I heard many times that the frontal lobes reorganize themselves during adolescence. This is used to explain why teenagers make bone-headed decisions, and as a caution to kids not to mess with drugs or injuries during this period. Your question set me to looking for some quick concrete information. There are many books on the subject, but I found an interesting page on the National Institute of Mental Health website. 

http://www.nimh.nih.gov/health/publications/teenage-brain-a-work-in-progress-fact-sheet/index.shtml

The NIMH paper reports research saying that there appear to be two major growth spurts of the human brain, one around the time of birth and another shortly prior to puberty. It further suggests that each of these growing periods are followed by a pruning process, while the brain decides which of the new connections are important and which are not. These processes are highly individual from person to person, and they say that it is difficult to make generalizations. Research suggests that the growth of white matter in the brain proceeds typically from the front to the back, at the time of puberty. ". . .studies have suggested that gray matter maturation flows in the opposite direction, with the frontal lobes not fully maturing until young adulthood." That it is followed, over a course of years, by a growth or development of myelin sheathing from the back toward the front. Myelin forms on the outside of neurons and enhances their performance.

Neurologists use functional magnetic resonance imaging (fMRI) to watch the brain non-invasively as it performs tasks. It is truly an amazing and exciting technology. Meanwhile I still advise teens to be careful with their brains.

Class Ring Physics Question

Why does a class ring, spun like a top on its stone, stands up with the stone on the top?

You have given this more thought than I have, and a quick Internet search has left me high and dry about the typical inverting toy tops, a wooden sphere with a short dowel. I am also striking out with "class ring physics." I have sent a note to Jostens, one of the major purveyors of these rings, and I will let you know if they have any help to offer. [No reply]

Meanwhile, I do have some information to stir into the recipe.

Unless you went to MIT* or some other institution that doesn't use a stone, the stone has a third or less the density of the metal. (Quartz D= 2.7, Silver D= 10.5, Gold D= 19.3, many gems are based on quartz or closely related to it.)

As to how it would behave in microgravity, I can only speculate, although with all the  personnel who have flown to space, I suspect someone must have tried spinning her or his ring! It would be difficult, if not impossible, by hand, to get a pure spin on the axis of the stone or axis of symmetry of the ring. Therefore, I would wager that the ring would appear to rotate as you have described, but I would also be willing to bet that it won’t exhibit any preference for a particular orientation. NASA had a "Toys in Space" program, and I hope they took a tippy top.

Having just remembered the expression, "Tippy Top," I discovered this amazing page of information. http://www.yorku.ca/marko/ComPhys/TippyTop/TippyTop.html  I am afraid that you will have to explain to me what it means beyond about the third paragraph. :)

*My dad's "Brass Rat" is too worn to spin well, but he doesn't recall seeing MIT class rings stand up as you describe.

Little Boy Replica Question

We were asked why we replaced our Little Boy artifact.

Our current Little Boy model is actually a very faithful fiberglass and metal replica of the bomb that was dropped on Hiroshima. It is even ballasted with concrete to simulate the weight of the original. As such it is a more faithful model than the one it replaced.

Our original Little Boy was built after WWII for the US' nuclear weapons program, but it never carried any active materials, nuclear or high explosive. It was a second generation gun type weapon, and in some ways different from Little Boy. I am told that after 9-11, the National Nuclear Security Administration felt that the several of these objects that were on display (including one at the American History Museum at the Smithsonian Institution) contained more information inside them than the NNSA was willing to share in case of a theft. Some of the weapons they pulled have had their guts removed and are back on display, but the BSM chose instead to have a more faithful appearing reproduction fabricated.

Human Battery Question

You ask if there would be an electrical current if one of the plates were made of rock.

The short answer is no. Unless the rock is a very rich metal ore, rock will not conduct electricity well enough to allow a current.

When we put our hands on the metal plates, we complete a circuit, electricity flows from one plate to the other through our arms and body, and then back to the original plate through the micro-ammeter via wires. The current, which is very small, is caused by the different way the metal plates react to the moisture in our skin; one collects electrons and the other gives them up. Any two different conductors will serve the purpose. Ours are copper and aluminum; but they need to be conductors in order to complete the electrical circuit. The human body is a pretty good conductor.

Most rocks are insulators and can't complete the circuit.

Mushroom Cloud Question

Any explosion above a pretty small scale is likely to result in a mushroom cloud. An explosion occurs when a dense substance, usually solid or liquid, turns into a gas. The result is an enormous and fast increase in its volume, or size. Most explosions we are familiar with involve the production of heat. An atomic explosion is no exception. The hot gas, being less dense than the air around it, rises.

As the gas rises, it also continues to expand. The expansion and contact with cool air above the rising column causes the gas to cool. Hot gas from below is still trying to rise, and the cooling gas above it is in the way. This makes the cloud of smoke and dust spread out like the head of a mushroom. This structure is a vortex, like a smoke ring (Do people even make smoke rings any more?) with a torus, or doughnut, shape, the gas on the inside traveling up, out across the top, down the sides, and back up through the center.

Volcanoes can make mushroom clouds if they explode with enough force. The volcanic explosions that formed the Valles Caldera behind Los Alamos must have made spectacular mushroom clouds! According to Wikipedia, the largest mushroom clouds ever seen were on Jupiter during the impacts of Comet Shoemaker-Levy.