# Angular Momentum Makes Science FUN!

Woohooo! Science is fun! What you just saw is a classic demonstration of something called Conservation of Angular Momentum. This concept can be difficult to grasp, and is certainly a mouthful. Let’s break it down and see if we can understand what is happening!

A good place to start is the last word. Momentum is a mathematical quantity (technically it’s a vector) that can be calculated by multiplying something’s mass by its speed. Something that is heavy and fast has more momentum than something heavy and slow, light and fast, or especially light and slow.

Angular momentum is just momentum in a circle. This can be calculated by multiplying the momentum by its distance from the center of the circle that it is traveling in, or the radius. This is a little harder to understand, but if two things have the same momentum, the one that is traveling in a bigger circle will have the larger angular momentum. If two objects are at the same radius, the one that has the larger momentum has a larger angular momentum.

Why do we care? Mostly because angular momentum is a conserved quantity. You may have heard of Conservation of Energy before, a principle that simply states that energy can not be created nor destroyed. Conservation of Angular Momentum works the same way, and as we saw that it is basically mass tim es velocity times radius. Since mass isn’t something that we see appear or disappear very often, this really only means two things can change: its speed and the size of its circle.

The total amount of angular momentum has to stay the same. That’s the conservation part. So if you have something at a large radius and bring it closer, the speed has to go up to compensate. There are several good real life ways to see this. Watch the beginning and the end of a game of tetherball, and you should notice a dramatic difference. Similarly, figure skaters often go into dizzying routines where they bring their arms in and start to spin faster and faster!

This same thing can happen on a cosmic scale. One cool example is in our solar system. Jupiter is well known for being the biggest planet, but it also has the shortest day at just under 10 hours long! Jupiter formed from a huge gas cloud that was slowly spinning. As it collapsed into a planet, it had to spin faster because of conservation of angular momentum! Even cooler: this process is still happening. Even as we speak, Jupiter is still shrinking at a rate of about 2 cm per year, which isn’t a lot…but still means that the planet is speeding up!

The animation on the left shows a star forming by a similar process. Credit Matthew Bate

There is another much more exotic example: Neutron stars! We have written about these before, which you can read here. In short, neutron stars form when stars much larger than our sun explode and leave a collapsed core behind. These neutron stars are only about 10 miles in diameter; a stark contrast with our sun’s 800,000 mile diameter. Stars spin too, and you can probably see where this is going. Even though the sun rotates just over once a month, conservation of momentum would demand that it spin faster if it were to shrink in such dramatic fashion. As a result, slow neutron stars spin about once every 8 seconds. Fast ones, known as pulsars, can spin around at over 700 times per second! Just be glad that doesn’t happen in the chair!

Neutron stars don’t look like much, but usually reside within these cool molecular clouds that were part of their stellar outer layers during their previous phases. Credit NASA/ESA/HST

# 10 Things You Didn’t Know About Nylon

Nylon was patented on February 16, 1937 by Wallace Carothers. Almost exactly one year later, it hit the shelves in the form of toothbrush bristles. In the intervening years, it has been used for packaging food, building car engines, clothing, fishing line, 3D printing… it even went to the moon! But just what is this multipurpose material?

Buzz Aldrin and the Nylon flag that they brought to the moon on Apollo 11. Note the rigid support rod at the top to support the flag in the near vacuum conditions. Credit NASA.

This is a model of Nylon 6,6; the most common industrial form. Each molecule is relatively small and simple, but they bind together into long chains when the negative nitrogen bonds to the positively charged carbon at the base of another chain, similar to metallic links in a traditional chain. These long chains in chemistry are called polymers, and nylon is the name used for a large family of slight variations on this idea.

Caption: Models of Nylon 6,6 chaining together. Red is oxygen, blue is nitrogen, black is hydrogen, and the carbon chain backbone is white. This process happens through a chemical reaction called condensation polymerization, because a molecule of water is produced when the two connect.

One reason that nylon is so useful is that it is a thermoplastic. Around 500oF it begins to liquify. This allows it to be drawn out into long very thin strings. Nylon fabric is just like normal fabric, except that it is made of these tiny thermoplastic fibers. As a synthetic material, it doesn’t mold. It is also waterproof and quick drying, making it ideal for a lot of lightweight outdoor clothing. With all of the rain we are supposed to get this year, this is a good time for appreciating nylon!

This nylon molecule model is made out of…Nylon!

# Chemistry on Valentine’s Day!

Puns abound in our Valentine’s day video, but we’re not about to let a holiday get in the way of talking about cool science!

These cool color changes are brought about by chemistry, specifically by a little number known as pH. While it is pretty common knowledge that this number indicates how acidic something is, understanding acidity is a little more complicated but surprisingly elegant! Let’s start by exploring a surprising fact about water! This precious little molecule known as dihydrogen monoxide has a secret: it doesn’t always stay as H2O! As these tiny water molecules collide, they sometimes exchange hydrogen atoms, like this:

Animation by Dr. Walt Volland

This is an equilibrium, meaning the reaction is constantly going both directions. The H30+ ions are called hydronium and cause water to be more acidic. The OH ions are called hydroxide and make water more alkaline or basic.

In distilled water, which doesn’t contain anything else, this happens so that each of these has a concentration of 10-7, which is very very small. pH is just the negative power of the concentration of hydronium in this concentration, so distilled water has has a pH of 7, which is neutral. All of this is based on the equilibrium in the animation above.

Conveniently, the concentrations of the two always have their powers add up to -14. This means if you add a bunch of acid to some water to get a pH of 1 (which would be a concentration of 10-1), the concentration of the OH- ions would be 10-13. This should make some sense: If you pour a bunch of acids and bases together, you don’t end up with something that is an acid and a base. Instead, they combine to form water, until there is a little of one left over.

Unfortunately, just looking at a pool of water isn’t enough to tell if it is acidic or basic, since our eyes can’t see the tiny ions that are responsible floating around. Instead, we used something called Universal Indicator, which changes color depending on the pH of the solution it is in. Those colors can reveal what the solution is like by using the chart below.

The preliminary blue heart matches up with about a pH of 10. Since this is greater than 7, we know the solution is basic (or alkaline). Next, a spritz of vinegar is added, resulting in a change of heart. The color shifts to a pinkish color, which is on the acid side of the scale.

Universal indicator is not the only way to see pH. The second heart starts with a clear liquid, which actually contains an indicator called phenolphthalein. This indicator remains clear at all pH’s above 8.2, and between 10 and 13 it changes to a very holiday appropriate fuschia! To get this result, we simply added a dilute ammonia solution!

Happy Valentine’s Day!

# What Is Electricity?

An atom consists of three types of building blocks: protons, neutrons, and electrons. Protons and neutrons stick together in the center, or nucleus, of the atom, while electrons whiz around the outside at breakneck speed. The parts of an atom are incredibly tiny and surprisingly spread out. Let’s consider the simplest element of them all: hydrogen. Its atom contains just one proton and one electron. If the proton were two football fields across, the electron’s orbit would be about the size of the earth! Between the central proton and the faraway, speeding electron? A whole lot of empty space.

Protons are defined as having positive charge and electrons as having negative charge. Neutrons are– you guessed it– neutral. It’s a law of nature that like charges repel each other and opposites attract. Put a lot of negatively charged electrons in one place, and if they’re free to move, they’ll spread out as much as possible. Getting a lot of negatively charged particles in one place and giving them the ability to move is one common way to generate electrical power. Batteries create a surplus of electrons via a chemical reaction.

Large-scale generators get electrons moving using a different trick: electromagnetic induction. Moving magnetic fields spur charged particles into motion, creating electrical activity in wire coils.

Whether the source is a battery or a generator, moving electrons don’t provide useful energy unless they have the right path to follow: a closed loop through a device to be powered, such as a light bulb or motor. This path is called a circuit. Light switches, power buttons, and outlets all provide different ways of closing a circuit so that electrons can flow.

The simplest circuit is a single loop, but in practice, it’s useful to be able to power several devices with just one source. Consider a string of lights. There are two ways to light them all at once: by linking everything together in one long chain, or by connecting the power source across each output element independently. We call the single chain approach a series circuit and the independent connection strategy a parallel circuit. Each method comes with tradeoffs in voltage and current. As you can see in the video, this affects the brightness of our two test bulbs!

These light bulbs are connected in parallel. The voltage through each one is equal to the whole voltage of the attached battery.

Written By: Caela Barry

# Godzilla El Niño and You

You’ve probably heard a lot about this year’s El Niño, which is predicted to be the biggest since 1998. What exactly does that mean? This global weather phenomenon has complex and far-reaching consequences, but the mechanism behind it is simple!

Let’s start by considering normal weather patterns in the Pacific ocean. Water near the surface is heated by the sun. This warm upper layer floats on top of cooler, denser ocean depths, resulting in a dramatic temperature barrier called the thermocline. At left, we’ve modeled the thermocline in a plastic container: warm water is colored red, and cool water is colored blue. Because the warm water is less dense, it floats.

In a non-El-Niño year, equatorial trade winds blow strongly from east to west, moving surface water along with them. In the Pacific, these winds push the warm upper layer of the ocean towards Australasia. Cooler water wells up in the east to fill the space left by the warm water moving out. As a result, the thermocline sits at a slant. Surface temperatures are normally warmer in Indonesia than Ecuador by about 45 degrees Fahrenheit.

This tilt in the thermocline is responsible for the typically wet climate in Southeast Asia and relatively dry conditions along the Americas’ equatorial Pacific coast. Warm water near southeast Asia and Australia evaporates and blows inland, resulting in lots of rain. Cool, nutrient-rich water from deep in the ocean sustains whole ecosystems in the eastern Pacific, but doesn’t generate much precipitation.

During an El Niño year, the thermocline shifts, resulting in warmer than normal surface temperatures in the eastern tropical Pacific. Animation credit: NASA Goddard

In an El Niño year, trade winds weaken. With nothing to hold it in place, warm surface water sloshes eastward towards the Americas, leaving the western Pacific cooler than normal. Australia and southeast Asia experience drought conditions. The equatorial Americas see unusually intense precipitation, which comes in the form of coastal flooding in some regions and severe winters in others. Since warm water doesn’t vacate the eastern ocean, cool water can’t well up to replace it. Coastal ecosystems that depend on deep-ocean nitrates and phosphates struggle, undermining economies that depend on industrial fishing.

El Niño is defined as unusually warm surface water in the eastern tropical Pacific, but its consequences extend around the world. North of the equator, the western U.S. sees intense winter storms. Across the ocean, unseasonably dry weather disrupts agriculture in Indonesia, Australia, and even South Africa.

Although the thermocline’s oscillation isn’t precisely predictable, it is part of an ocean temperature cycle that repeats every two to seven years. So, yes, it’s going to be an intense season! That said, this is nothing that hasn’t happened before, and we’ll likely see a new “Godzilla” El Niño in another decade or a few.

Written by: Scott Alton, Caela Barry

# Liquid Nitrogen Rocket! Here’s Why.

Every step you take is a tiny study in rocket science! Lift one foot and push backwards on the floor with the other. The floor exerts an equal and opposite (forward) force on the planted foot, and your body moves ahead.

Equal and opposite force pairs are everywhere. Stand on an ice rink and push against the wall; the wall pushes back, and you slide away. Sit in a chair, and your body pushes down against the seat, which pushes up with precisely the same force– if it didn’t, you’d fall! Birds push down on the air with their wings, and that air pushes their bodies upward.

Image credit: NASA

The same process propels a rocket in motion. Thrusters shove fuel out behind a spacecraft. The fuel pushes back, equally and oppositely, and the rocket moves forward. Actual rocket fuel is accelerated backwards by explosive chemistry. We can model spaceflight in the lab with any expanding fuel whose flow can be directed.

Liquid nitrogen boils into gaseous form at -321 degrees Fahrenheit. This means if it is warmer than that, it will turn from a liquid to a gas that takes up 694 times as much space. We trapped this nitrogen in a ping pong ball with two very small holes on the sides. When the nitrogen expands, this is the only way out! The holes are positioned in opposite directions on opposite sides, and as the gas is directed out of these makeshift nozzles, the equal and opposite force due to Newton’s Third Law accelerates it to truly incredible speeds!

Actual rockets are meticulously designed for optimum aerodynamics and equipped with fins for stability. Ping pong balls are pretty much the opposite of that! Our tabletop rocket is all over the place. With some structural modifications, more stable motion might be possible. For now, though, we’re just having fun with this nitrogen-powered turbo top!

# Bernoulli’s principle: Supercells & Airplanes

In strong supercell thunderstorms, wind moves upwards at over 90mph. Fast-moving air creates an area of low pressure, or a partial vacuum. Nature abhors a vacuum, so nearby air rushes in to join the updraft. This skyward flow can be powerful enough to suspend grapefruit-sized hail.

The tendency of a speeding air current to suck in surrounding air molecules stems from the same idea that gets airplanes off the ground: Bernoulli’s principle. The faster a fluid moves, the lower its pressure. Airplane wings have more pronounced curves on the top side than they do on the bottom, so air follows a longer path, and moves faster, above the wing than below. Higher pressure from underneath the wing translates to an upward force– that’s lift.

Aircraft and thunderstorms provide some of the most dramatic examples of this concept, but it’s also surprisingly easy to explore at home or in class. All you need is moving air and something to reveal its motion! A long, narrow bag and a working set of lungs are the perfect tools. Try to blow up eight feet of plastic bag like you would a balloon and you’ll find yourself struggling for air. Instead, hold the bag’s opening ten inches from your face, and blow into it across the gap. What’s the difference?

When your breath travels through a volume of air before reaching the bag, Bernoulli’s principle does most of the work for you! Your exhalation is a fast flow, so it creates a sink of low pressure moving into the plastic bag. Nearby air rushes in to fill the partial vacuum, and before you know it, the bag is fully inflated.

Firefighters clearing smoke from buildings take advantage of Bernoulli’s principle in almost exactly the same way. A fan placed directly in a door or window will move air through the structure. Set the same fan a short distance back from the opening, and it pulls in more air, getting the job done faster.

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