Monday, July 29, 2013

Update

 
I apologize for not writing much lately. I've spent the past 2 weeks traveling to/from and attending a volcano conference and acoustics workshop. I have some great pictures with some great science, so hopefully I'll be able to find the time to start posting more regularly again. Thanks for reading!

Saturday, June 22, 2013

How Do Animals Dry Themselves by Shaking?


** Apologies on not posting for a while. I hadn't noticed how long it's been! **


This is not a photo (!) but a video that I came across a couple years ago. The authors from Georgia Institute of Technology applied some basic physics to determine how quickly an animal has to shake in order to dry itself. Here's their website where you can find a more detailed description of their study, including the resulting published paper. Being a fan of applied physics, I really enjoyed this, especially because it is simple enough for an introductory physics student to understand.

The basic idea is to look at the forces acting on the water droplets in an animal's fur. The force exerted by air pressure and water tension work to hold the droplets in the fur. By shaking, the animal produces a force that acts to move the water droplets outward. This is called the centripetal force. When you are next in a car, notice how you move when the car makes a turn, especially if it's a quick turn. You should feel your body moving away from the pivot point. In other words, a right turn will cause you to move toward the left side of the car. This is also the centripetal force at work, and it occurs whenever a body is rotating.

The centripetal force produced is proportional to the angular frequency, or speed, of the rotation as well as the distance from the pivot point to the object that is rotating. For our rotating mammals, the pivot point is the center of the body and the object, the water/fur. Therefore, the distance is just the radius (or half the thickness) of the mammal's body. The conclusion that smaller animals must shake faster makes sense because of this proportionality.

Let's introduce the actual equation for rotational motion and centripetal force: Fc = m*r*w^2, where Fc is the force, m is the mass of the water droplet, r is the radius of the animal, and w is the angular frequency with which it shakes. Assume we want to keep Fc constant - that water droplets always require the same amount of force to remove them. Since a smaller animal has a smaller radius r, it will require a larger rotating speed w to compensate for the smaller r. Larger animals with larger radii, can shake more slowly to produce the same centripetal force as their smaller companions.

Here's a challenge: Next time you're at the beach or get out of the shower, try to shake yourself dry. Can you do it? If so, what do you think your shaking speed (oscillations per second) is? If not, what makes you different from the mammals that do dry by shaking?


References

Wet mammals shake at tuned frequencies to dry by Andrew Dickerson, Georgia Tech

Thursday, May 23, 2013

Why Does the Moon Have Phases?

** My apologies on the lateness - it's been a busy month. **


Full moon. Crescent moon. New moon. There's no doubt you've previously heard these or even talked about them. Perhaps you even know the less common or more specific phases of the moon. Yet, there still seem to be misconceptions about the moon's appearance. I've already covered why the full moon can seem so large and why the moon can appear orange when it's on the horizon. Here's some more information about the moon's appearance:

The Moon's Orbit & Apparent Size
Let's start with some important background knowledge. The moon orbits the Earth on a nearly, but not perfectly, circular path. As a consequence, there are two points where the moon will be at its closest (perigee) and farthest (apogee) distance from the Earth with every other point somewhere in between. The difference in distance between the two extremes is enough to be noticeable, at least with pictures. It might be a bit difficult to see the difference by just going outside and taking a look. Another neat thing about the moon is that it's tidally locked in its orbit. For every one orbit around the Earth, it will rotate exactly one time. The result is that someone on the Earth can only ever see one side of the moon. If you compare the two pictures above, you'll notice that the dark lunar mare near the bottom right corner of the crescent moon is the same as that on the right side of the gibbous moon. On the flip side, there is one side of the moon that we can never see from the Earth. This is where the "dark side" of the moon comes from. The dark side of the moon isn't dark in the true sense of the word. There are times when the sun shines on it. However, it's considered "dark" because people were never able to see it until lunar orbiter spacecraft were built and sent into space.

Moon Phases
Unlike the sun, the moon does not emit light on its own - we only see it because of the sunlight that it reflects. Like the Earth, the moon will only have sunlight illuminating half of it at a time. The half that is illuminated is not always that half of the moon that faces the Earth, which leads to the phases. I'll be a bit more specific with a few examples to help clear up any confusion. When the moon is full, the half that is illuminated is also the half that faces the Earth. The moon is on the opposite side of the Earth than the sun, so the sunlight illuminates the half that faces us. The opposite of this is the new moon. In this case, the moon and sun are on the same side of the Earth, so the half that is illuminated is the "dark" side of the moon. 

This case also brings up another interesting point: the new moon should be located close to the sun in the sky. In other words, we see the new moon during the day rather than at night. It doesn't disappear from the sky or become dark. Instead, it is just visible at a different time of day. 

Let's leave this little detour and get back to the phases. The last example that I'll give is the half moon. To see a half moon in the sky, the moon would be 90 degrees from the sun. If you have trouble visualizing that, picture it this way: if the Earth is a clock, the sun will appear near the 12 and the moon will appear near the 3. For this phase, half of the "dark" side is illuminated and half of the side that we see is illuminated. To get the rest of the phases, we can just rotate the moon around the Earth. The farther it gets from the sun, the closer it will get to a full moon. As it moves from full to new, a smaller and smaller section (visible to us) will be illuminated. If you need some extra visualization, you check out this chart or try the activity listed below.

Eclipses
I'm sure most people are well aware of solar eclipses, but did you know that the moon can be eclipsed, too? A lunar eclipse happens when all of the moon, Earth, and sun are aligned with the Earth in between the other two. As the moon passes behind the Earth, it moves into the Earth's shadow. Since the moon appears lit due to reflected sunlight, being in a shadow will make the moon appear darker. At the start of the lunar eclipse, a growing fraction of the moon will appear blacked out. As more and more of the moon moves into the Earth's shadow, it starts to turn a dark orange color. The next total lunar eclipse visible for the Americas will be on October 8, 2014, so get your binoculars ready.

Try It at Home
You can see for yourself how this process works with just a (solid) ball and a flashlight. Begin by placing the ball on a table with the flashlight pointed at it from a couple or so feet away. Walk around the table with the surface at eye level and watch the appearance of the ball. When you're opposite the flashlight (sun), the ball (moon) appears as a dark, "new moon" phase. Go around to the same side as the flashlight, and you'll see a full moon (or rather full ball?). As you walk around, you'll see the phases of the ball changing just like the moon. However, the moon orbits the Earth, not the other way around. For a more realistic demo, place a larger ball (or globe if you have one on hand) a few feet from the flashlight. You can simulate the moon and its phases by moving the small ball around the large one. Place a "person" on the large ball. How does the small ball's phase change from your person's point of view as it moves around in its orbit?

Keep in mind that the moon's orbit is slightly tilted. This means that the moon and Earth rarely appear directly between the sun and the other. By aligning all three, you are creating a solar (moon in middle) or lunar (Earth in middle) eclipse.


References & Further Reading
Volcanism on the Moon - by Robert Wickman
The Lunar Orbiter Program - by Lunar and Planetary Institute (NASA)

Sunday, April 28, 2013

Water Waves


Chances are you've been to a beach before, and it's quite likely that there were waves whether you were at the ocean, a lake, or some other sizeable body of water. If you've ever watched the waves for a while, you might have noticed that they tend to arrive parallel, or nearly so, to the shoreline. Even if they are at angle when they are far away, they still reach the shoreline nearly parallel. Why? Another thing noticeable to anyone who has watches waves, and is visible in the above photo, is the crashing and "breaking" of waves as them come towards the shoreline. How does that happen?

The first question can be answered by fluid dynamics. In shallow water, that is at a depth which is smaller than the wavelength (measure of distance between wave crests), the speed of the waves depends on the water depth and gravity only. Specifically, the speed is equal to the square root of gravity times depth. This tells us that waves will move slowest in the shallowest water. Let's assume that there is a nice downward slope as you move away from the shoreline. Start by imagining a wave that begins parallel to the shore. As it moves closer, the shallower water will cause it to slow down. Now, what happens if the wave is rotated so that it starts at an angle to the shoreline?

If you think of the wave crest as a straight line, one end will be closer to shore than the other if it's at an angle. The far end is in deeper water and will travel faster than the shallow end. This causes the wave crest to turn until both ends are at the same depth or it hits the shore (whichever comes first). If you're having trouble imagining this scenario, try thinking about an axle with a wheel on each end. If you move both wheels at the same speed, the axle will move in a straight line. What happens if you hold one wheel stationary (more or less) and move the other? The moving wheel will rotate around the other one. Now, allow both wheels to turn but move one a little faster than the other. The axle will move forward while also gradually turning. This is analogous to our wave scenario.

That's one question answered, so on to the next one: what causes waves topple over? It may seem strange but this answer relies on the same principle as the first one. In this case, we can no longer treat the wave crest as just a line; it does have a finite width, after all. Since the back of the wave is in deeper water than the front, the back will travel faster and eventually catch the front. The water particles will accumulate behind the crest until it reaches a point when the added water forces the wave to topple over. If the wave is large enough, you may notice how the top rolls over the bottom. As water piles up behind the wave, the water becomes even deeper than the water in front. Consequently, the water in back moves even faster, particularly that on the surface. This helps the top of the wave crest to roll as the wave topples over. Keep in mind, however, that this is only valid for waves in shallow water. It's not often that you see breaking waves in the middle of a lake, partly for this reason. When you do see them, the wind is generally very strong and helps force them to topple over.

Fortunately, these are easy things to go see for yourself, so get out and enjoy the coming summer. Maybe the next time you find yourself at a beach, you'll pay a little more attention to the waves.


References
Waves in Fluids (video) by National Committee for Fluid Mechanics Films (1960s; more here)

Sunday, April 14, 2013

Sonoluminescence

I believe the credit for this one goes to my lab partner.

At first sight, those three little pin-pricks of light may not seem very spectacular. However, the phenomenon that creates them (sonoluminescence) is really quite interesting. Some scientists once believed that the light was created by the ever elusive nuclear fusion, although this idea has since been almost completely rejected. We can get to the basic idea by breaking apart the name, sonoluminescence. The "sono" refers to sound, while "luminescence" is the glow. In essence, the lights are created by sound.

In the experiment above, we start with a rectangle container filled with purified water. The metal cylinder at the top that sticks into the water is a transducer; in other words, it will create the sound for us. Sound is a pressure wave; when it travels through the water (in this case), it causes the water molecules to oscillate back-and-forth causing regions of compression, where molecules are pushed together, and rarefaction, where particles are pulled away. The sound that we used was a higher pitch than humans can hear, known as ultrasound. If the frequency of the sound (that is, how many times it oscillates per second) is chosen correctly, a standing wave will form in the water. If you and a friend each hold one side of a rope and shake it in sync with one another, a standing wave will form. You'll know when one forms because there will be at least one point on the rope that doesn't seem to move. The stationary point is called a node. Between each node is an anti-node, the point that has the most motion. The anti-node is what we're interested in.

The next step is to make a very small bubble very near the anti-node. It is a difficult task, but if done correctly, the bubble move into the anti-node and get trapped there. Because the anti-node is where the wave motion varies the most, this point will rapidly oscillate between a high pressure state (compression) and a low pressure state (rarefaction). During the high pressure state, the air in the bubble will be squeezed into a smaller volume. The opposite happens during the low pressure state; that is, bubble's volume grows and the air inside can spread out more. This should help to explain why it is so difficult to trap a bubble. If you squeeze or stretch it just a little too much, the bubble will pop forcing you to start over.

So, we have a bubble in water that is changing volume due to being trapped in an anti-node of a sound wave. Now that the experiment is set-up, we can move to our initial question: why does the bubble light up? This is a difficult question to answer and, frankly, the exact answer remains a mystery of physics. However, we can make measurements that give us some insights into the process and allow us to speculate on what it could be. The light is emitted when the bubble is squeezed. At this point, the volume that air molecules have is very small. When the bubble is squeezed, the air molecules heat up. In essence, this heat causes the molecules to emit light. What isn't known is the process through which the light is emitted. The most accepted idea is that the heat energy pulls the molecules apart. When the molecules re-form, they emit light. However, this does not explain the spectrum (or variety) of light emitted. As the experiments continue, scientists hope to one day find the answer.

While this is a very neat experiment, it might not have much practical use. However, sonoluminescence does occur in nature. Have you ever heard of the beautiful mantis shrimp? Perhaps you've seen the comic about it that's been getting passed around lately. The mantis shrimp can close its claw so quickly that it creates a high enough pressure wave to form a cavitation bubble that can glow by sonoluminescence. Just be careful if you go looking for one. Not only do they create a large enough pressure wave to kill their prey, but it is even strong enough to break through the glass of an aquarium.


References and Further Reading
The Sonoluminescence Process by the American Institute of Physics
Sound is a Pressure Wave by The Physics Classroom
Sonoluminescence: Sound into Light (Scientific American article) by Seth Putterman
Mantis Shrimp by Chesapeake Bay Program
Why the Mantis Shrimp is my new favorite animal by The Oatmeal

Sunday, March 31, 2013

Schrodinger's Cat


Up to this point, I've covered mostly topics involving geology and light. To shake things up a bit, this post will cover something a bit different - quantum mechanics. Generally speaking, quantum things are too small to photograph without a powerful microscope. However, there is a good macroscopic analogy for one of the most important quantum processes. The Schrodinger's cat paradox was originally used by Erwin Schrodinger to show how silly quantum mechanics seemed when applied to the macroscopic world in which we live. In contrast, his cat paradox has turned into one of the most widely used analogies for explaining the quantum phenomenon of superposition.

If you're not familiar with the Schrodinger's cat paradox, here's how it goes:
(Note: Do NOT try this at home. This is a thought experiment only, not something actually carried out. Please do not needlessly harm animals.)

Start with a solid box. Inside the box is a vial of poison attached to a special device that contains an unstable atom. This unstable atom has a 50% chance of decaying within an hour. If the atom decays, the device will trigger a hammer to smash the vial of poison. Let's now place a cat inside the box and then close the box. If we wait an hour, there is a 50% chance that the device was trigger and the vial smashed. If that happened, we would expect to see a dead cat upon opening the box. However, there is also a 50% chance that nothing happened and the cat is still alive. So, without opening the box, is the cat dead or alive after an hour of waiting?

As long as we don't observe the cat by opening the bow and looking at it, we can say that it is in a state of being both dead and alive. Now, this doesn't make much sense. Obviously, an animal can only be dead or alive, not both. This is why it is a paradox and why Mr. Schrodinger used this as an example of how ridiculous the quantum theory of superposition seemed. Now, what exactly is superposition and why is it important?

From the cat paradox, you may have figured out what superposition is. In simple terms, superposition means that a quantum particle, such as an electron, can be in multiple states at the same time. There is one catch though - if you try to measure or observe the particle, it will assume only one of the states. Going back to the cat paradox, once you open the box and check to see what happened, the cat will be either alive or dead. You can no longer say that it exists in both states.

One consequence of this is that the act of measuring a system can affect how the system is behaving. This is something that rarely happens in the macroscopic world. If you use a ruler to measure the length of a pencil, the act of measuring will not change the length of the pencil. Its length will be exactly the same before and after you measure it. In the quantum world, however, things are different. Because particles are so small, even light can easily affect the properties of a particle. For example, when light collides with an electron, it can pass momentum to the electron and change its speed and/or path of motion. This consequence becomes significant when trying to do things on a quantum level, such as building a quantum computer. These computers use the fact that an atom can simultaneously be in multiple states to speed computations by doing more than one at the same time. However, to get the computation results you would need to interact with the atoms. This causes them to lose all but one of their current states and breaks the multiple-state basis of the computer.

There are experiments that modern day scientists can use to test the theory of superposition. Even though it was questioned back in the 1930s when the cat paradox was first introduced, it is now a well accepted theory. That said, from Mr. Schrodinger's days until now, one thing remains the same - the quantum world remains a very strange one that few people can even begin to fully understand.


Further Reading
Schrodinger's Cat Comes into View - By physicsworld.com
No. 347: Schrodinger's Cat - By John Lienhard at the University of Houston
Another step toward quantum computers: Using photons for memory - By Eric Gershon, phys.org
What Is Quantum Superposition? - By the Science Channel