# The Slinky: Mystery of a Childhood Toy

Everyone loves a Slinky, especially scientists! Most people know them as cool springy things that can go down the stairs or end up in a giant tangled mess. We want to focus on something else: what happens when you dangle a slinky from the top until it is fully extended, then drop it?  You might expect the whole slinky to fall to the ground. As you can see in the video, it’s a little more interesting than that. The bottom of the slinky stays completely motionless until the top of the slinky catches up. But why?

The slinky isn’t breaking physics or ruining science. It’s really just a loose spring. The more a spring is stretched, the more it pulls back towards the center. When the slinky is extended vertically like this, gravity pulls down on every part of it, including the bottom. Before the drop, we allow the slinky to hang until it stops moving, which means that the gravity pulling down on it is exactly countered by the spring force pulling back up.

The whole slinky is pulled down by gravity with the same acceleration. Both ends are also pulled towards the middle due to its springy characteristics. The top is pulled down by gravity and the spring, so it falls extra fast. The bottom, even as the slinky is dropped, is still being pulled up by the spring. With the bottom going nowhere, and the top going faster, it all averages out. The whole slinky together is falling at the exact rate that gravity dictates.

There is a bit more to this explanation. The slinky actually shows a wave and the transmission of information as well. The information from the top (“we’re falling!”) needs to get to the bottom before it can take effect there. You can see this wave– called a compression wave– if you watch the top of the slinky in the gif below. The spring clumps up as the fast-falling top part catches up with the parts below it.

There is nothing special about our Slinky. It doesn’t need to be rainbow colored or giant, any old Slinky will do the trick. Don’t believe us? Try this one at home! And when you’re done, take it to the stairs!

# Why on Earth Can Oranges Spray Fire!?

Oranges are fruits that grow on trees. They are made up of almost 90% water by weight. This should pretty clearly mean that they are not flammable. However, there is something going on. Take a look:

Inside of the peel is a special oil called D-Limonene that is quite flammable. What reason could an orange have for harboring a fiery fluid in its skin? Well, it has to do with where baby oranges come from.

That’s actually a tad misleading. Oranges contain the seeds of orange trees. The orange itself is another orange tree ready to grow, but the original tree doesn’t want to get too crowded. This orange needs to go off and grow somewhere else. To do that, it simply has to be delicious. If the orange is eaten and then pooped out elsewhere, it can grow into a nice successful orange tree and produce oranges of its own. The problem, of course, is that oranges are too tasty!

It’s not just the desired animals that will nicely eat the oranges and carry their seeds to a new home, it’s also pests like bugs, which are… less qualified for this important task. As a defense, oranges contain the previously mentioned D-limonene, which drives away insects and mold. This makes the orange more likely to be devoured by its target audience, which leads to more orange trees and then to more oranges!

Interesting side note: most oranges in grocery stores are seedless! In fact, the one in the video is seedless. Instead they are grown through a bizarre process called grafting, where a piece of seedless orange plant is stuck to another and they grow together. Image from K-Mega Farms.

# DIY Optics Experiment

Ever tried burning wood or pine needles with a magnifying glass? It works best when you adjust the glass to create a small, bright spot of light. After its 93-million-mile journey to Earth, sunlight isn’t ordinarily hot enough to start a fire, but the magnifying lens focuses a large cross-section of light rays into a single intense point. This kind of lens is called convex.

A glass of water is essentially a convex lens. Just like sunlight passing through a magnifying glass, an image traveling through a cylinder of water is focused into a single spot some distance away. The rays don’t stop there, and as they continue, their paths cross. The part of the image that started on the right ends up on the left, and vice versa!

This is an easy trick to try at home! All you need is a clear glass or jar and an image to flip. If you don’t see the expected results right away, try putting more distance between yourself and the glass– the image will only appear backwards if you’re past the point where the rays of light meet and cross.

A cool astronomical tidbit: Another thing that bends light is gravity. This allows it to focus light as well, and sometimes Earth will be on the receiving end of this. Unlike the fire starting focused sunlight, this is a useful tool that allows scientists to locate black holes and get unique views of faraway objects and is known as gravitational lensing. For a better understanding, check out the animation below:

The ring-like structure of the galaxy in the picture is actually quite common in space. Scientists can’t control when they can see or use gravitational lensing as it is the result of the positions of objects that are very far away, but these serendipitous events allow us to learn more about both of the objects involved!

Two gravitational lensing examples captured by Hubble. On the top is a nearly complete Einstein ring. On the bottom, the lensing alignment is slightly off, smearing the images of the distant galaxies to form a friendly smiley face!

Credit: NASA/ESA/Hubble

# The Science of Depth Charges

Imagine hitting a hanging punching bag. A significant part of the impact travels through the bag, causing it to swing back and forth. Now replace the punching bag with a down pillow. This softer, less dense target absorbs most of the shock instead of transmitting it. The pillow swings a little, but deforms a lot. The punching bag and the pillow illustrate the effects of pressure waves in water and air, respectively. Air is relatively squishy. Its molecules have some wiggle room, so it compresses and decompresses with ease. Water molecules, on the other hand, are packed together so closely that water is often called incompressible. Like the swinging punching bag, water moves with a pressure wave instead of attenuating it.

Above: the effect of a depth charge on a nearby ship. As the pressure wave travels through the hull, the ship flexes back and forth, weakening its structural components. Image credit: Naval Postgraduate School

If you’ve ever tried to run or throw a ball underwater, you’ve probably noticed that liquid creates a lot of drag on moving objects. Submerged shrapnel doesn’t travel far for exactly this reason. The primary danger from a depth charge explosion is the pressure wave itself, not the bits of debris it throws around. In World Wars I and II, naval ships detonated bombs near submarines to compromise their structural integrity and key instruments. Initial blast damage was rarely fatal, but the power of the underwater pressure wave was debilitating enough to force targets to surface, opening them to direct fire.

A person unlucky enough to be outside a submersible in the range of a depth charge would experience another gruesome difference between blast physics on land and underwater. Humans are denser than Earth’s atmosphere, so in air, pressure waves primarily bounce off of us. The pressure differential still has unpleasant effects on air pockets such as inner ears, but most of our body parts are safe from implosion. Humans and water, though, have similar densities. This means that an underwater pressure wave passes through a submerged person, causing catastrophic damage to vital organs. Image credit: UC Davis

Destructive as weaponized depth charges can be, mini versions are a lot of fun to play with (carefully) in the lab. Our submersible explosive is simple: a small amount of solid carbon dioxide and water are trapped inside of a film canister. You may also see pennies in the video. These were used to weigh down the canister, but did not otherwise contribute to the result. As the carbon dioxide sublimates (goes from solid to gas), it expands rapidly, popping the top off the container. This tabletop explosion is cool in air, but even more dramatic underwater, where the blast radius is clearly visible! Science is awesome!

# From Art to Action: Mythbusters 101

Great teachers know the classroom is a stage and leverage that knowledge to create exceptional experiences for students. Classroom audiences have the advantage of being able to interact physically as part of the learning process. If viewers happen to be on the other side of a TV screen, showmanship and energy have to compensate for physical distance. Who better to bring explosive, larger-than-life science to living rooms worldwide than a team of accomplished special effects artists? The rotating crew of guerrilla experimenters known as the Mythbusters is just that.

Left to right: Grant Imahara, Jamie Hyneman, Kari Byron, Adam Savage, Tory Belleci. Image credit: Discovery Channel

In their own words, they’re not scientists. The team’s collective work experience is astoundingly broad, with pre-Mythbusters gigs ranging from divemaster to welder to wilderness survival specialist to successful artist and beyond. Their special effects credentials include such franchises as Star Wars, The Matrix, and Terminator. In a 2010 interview, co-host Adam Savage calls the show’s dynamic cast of builder-experimenter-hosts “unencumbered by training.”

This 2009 explosion, designed to test whether it’s possible to knock a person’s socks off, unexpectedly broke windows over a mile away. Mythbusters replaced the shattered windows on the day of the incident. Image credit: Discovery Channel

Co-host Jamie Hyneman agrees: “if we knew what we were doing we wouldn’t be entertaining.” These experimenters aren’t teaching their expertise, they’re problem-solving and learning in front of an audience– and that’s a huge part of their appeal. Hollywood artistry and resources take viewers along on a wild crash-test of a ride. It’s raw curiosity armed with action movie appeal, contagious enthusiasm, and a whole lot of imagination. The Mythbusters are popularizing a pure, accessible form of science. Ask an interesting question, design a test, learn something new, and have fun doing it. This is exactly what kids need to see.

AstroCamp science question of the day: how big a cloud can we make if we pour all of our leftover liquid nitrogen from the dining hall deck into a tub of hot water? Answer: this big!

Ever cover the end of a garden hose with your thumb to turn it into a long-range water blaster? The same thing happens when wind is squeezed between mountain ranges. This is called the Venturi effect, and it’s how wind engineers make the most of their turbine placements. California’s desert valleys are home to America’s oldest wind farms because they function as natural wind tunnels.

The San Gorgonio Pass wind farm, just down the mountain from Astrocamp, is one of the oldest in the United States. Its operational capacity is 615MW. For more on how a windmill actually produces electricity, click here! Photo credit: Gregg M. Erickson.

Wind energy has seen massive expansion since it first gained traction in the 1980s, and it continues to grow. In 2014, U.S. wind farms generated over 180 million kilowatt-hours of electricity. That’s enough to power about 15% of domestic households. A recent Harvard study indicates that establishing a widespread turbine network across the central plains of the U.S. would power the nation many times over. Taking hydropower and other forms of electrical generation into account, Europe and China produce comparable levels of renewable energy, and many other regions are in hot pursuit.

Global renewable electricity production by region, historical and projected. Source: International Energy Agency.

Many of the engineers who will propel the world into a more sustainable future are training in today’s elementary classrooms. The skills they’ll need as future problem-solvers can and should be learned now. AstroCamp’s newest curriculum offering encourages young people to practice the engineering cycle in an environment that balances collaboration, friendly competition, and real-world relevance.

Their goal: Create, through several iterations of design and testing, a turbine to maximize power production from a model wind source, then team up with classmates to power a LEGO skyscraper!

# DIY Batteries

Batteries harness energy from the motion of electrons. How do we get electrons going? Chemistry! Most batteries use a simple oxidation-reduction (or redox) reaction to get electric charges moving. In a redox reaction, one material gives electrons to another. We call the donor the anode and the recipient the cathode. Strategic separation of the anode and cathode sets batteries apart from other redox reactions. Just like we can’t capture the power of the wind without putting a windmill in the way, we need to intercept the flow of electrons in a battery in order to get energy out. Completing a circuit around a battery gives electrons a path to follow and allows us to harvest power from their movement.

Galvanic battery. Electrons move through a wire circuit while ions flow through an  electrolyte solution between the anode and the cathode. Image credit: Simon Engelke.

In our tabletop battery, aluminum donates electrons to copper, creating positively charged aluminum ions (cations) and negatively charged copper ions (anions). If the copper anions build up at the cathode, their negative charge repels incoming electrons, and the redox reaction dies out. If we mix all the anions and cations together, their charges equalize, and electrons no longer flow. Ions on both sides of the reaction need a controlled escape path if a steady flow of electricity is to be maintained. This path is called a salt bridge, and it can be made of anything that contains ions and conducts electricity when in liquid form. Substances like these are known as electrolytes. Gatorade, Coke, lemon juice, and salt water all do the trick, although commercial batteries more commonly rely on alkaline pastes.

A tabletop battery with an aluminum anode and copper cathode. Electrons flow through the wires & multimeter. Ions flow through the Coke, which acts as a salt bridge.

This is an easy experiment to play with at home! A penny and a piece of a soda can make good electrodes  (buff the inside of the can with steel wool to remove the plastic coating and expose the aluminum). Experiment with produce items and non-toxic household liquids to see what works as a salt bridge. If your salt bridge is solid, like a lemon or a potato, just stick in the cathode and anode a small distance apart! When using a liquid electrolyte, hold your electrodes so that they both contact the liquid without touching each other. If you can, hook your DIY battery up to a voltmeter, ammeter, or multimeter so you can observe small electric flows, or try connecting several cells in series to light a small LED!

# Rockets & Newton’s Laws in 3 Gifs!

Rockets blast off from earth with a rumble and cloud of smoke on their way through the atmosphere to the vacuum of space beyond. This is something that is accepted today, but it hasn’t always been that way. Doctor Robert Goddard is known as the father of modern rocketry, but when he first postulated the current method of rocket propulsion in space, he was ridiculed. Even the New York Times published an article on the preposterousness of his ideas. Although rockets had been around for centuries, trying to use them in space was seen as ridiculous at the time.

The tools for understanding the basis of how a rocket works had been developed by Sir Isaac Newton hundreds of years earlier. Using Newton’s Laws of Motion, most of rocketry can be understood with relative ease.

His first law, often referred to as the law of inertia, states that an object in motion or at rest will remain in motion or at rest, respectively, unless acted upon by an outside force. This means that a rocket in space can turn its engines off and not slow down or even turn– unless an outside force, like gravity, acts on it. The bottle rockets in the video run out of fuel after a tiny fraction of a second, but they continue traveling upward until Earth’s gravity overcomes their momentum.

This variation on the classic “pull out the tablecloth” trick is a great example of Newton’s first law. Nothing pushes or pulls the tennis balls sideways, so inertia mandates that they don’t move horizontally. For more cool inertia demonstrations, check out our blog post here.

Newton’s second law is most easily understood as an equation.

The equation says that if you apply the same force to objects of two different masses, the lighter one will accelerate more than the heavier one. This shouldn’t be a surprise. If you kick a car with all your might, and then go kick a soccer ball with the same force, you will see (and feel) the difference! It’s easier to move or stop a light object than a heavy one.

The third and final law is what we like to call the law of interaction: for every action, there is an equal and opposite reaction. When a person stands, they push down on the ground, and the ground presses up equally on them. Imagine being on a frozen, icy lake, so slippery that you can’t walk. Normally, when you take a step, you push backwards against the ground, which in turn propels you forwards– an equal and opposite reaction. On an icy surface, you don’t have enough traction to push and move ahead this way. So, are you stuck? Not quite! Imagine standing on the ice and throwing a heavy rock. The rock exerts an equal and opposite force on you, pushing you in the other direction! Thinking this way makes a rocket easy to understand. Just like you can move across a slippery surface by throwing a heavy object away from you, a rocket can travel through the vacuum of space by shooting fire in the direction opposite its desired flight path.

A rocket doesn’t have to push against the air. It just throws fire out behind it, and the equal and opposite force thrusts it forward. As you can see, the basic idea behind rocket propulsion isn’t so complex after all.

So, what makes rocket science difficult in practice? Well, a rocket is basically a gigantic cylinder of explosive fuel with a nozzle to direct the explosion. The whole system moves at a very high speed. Without the right knowledge to control it, that’s a recipe for disaster! A rocket is also constantly launching its fiery fuel out behind it, so its mass is always changing. This makes calculations less than straightforward. As is the case in many fields, the core concepts in rocketry are simple. Complications arise when applying these ideas in real-world situations that require a lot of precision.

Blast-Off for the first space shuttle in 1981 from Kennedy Space Center. Credit NASA

### WELCOME TO OUR ASTROCAMP BLOG

We would like to thank you for visiting our blog. AstroCamp is a hands-on physical science program with an emphasis on astronomy and space exploration. Our classes and activities are designed to inspire students toward future success in their academic and personal pursuits. This blog is intended to provide you with up-to-date news and information about our camp programs, as well as current science and astronomical happenings. This blog has been created by our staff who have at least a Bachelors Degree in Physics or Astronomy, however it is not uncommon for them to have a Masters Degree or PhD. We encourage you to also follow us on Facebook, Instagram, Google+, Twitter, and Vine to see even more of our interesting science, space and astronomy information. Feel free to leave comments, questions, or share our blog with others. Please visit www.astrocampschool.org for additional information. Happy Reading!