Blog probablykevin

As a little programming project, I am working on a basic physics engine that is intended to eventually handle spherical particles, walls, and springs. Currently, I only have particles that are fully functional. This is an open-source project and it is hosted at http://code.google.com/p/simple-swing-physics-sim/.

Mr. Fullerton showed us this intriguing instrument in class and I just HAD to find it for myself. The Theremin is a musical instrument that changes its pitch and volume based on the electric field surrounding two rods. The player's right hand controls the pitch and his or her left hand controls volume. In AP Physics, we normally calculate electric field using with a value of 8.85x10^-12F/m, but this only works for a vacuum or air. When the player brings his or her hands closer to the instrument, the permittivity of the area of the field is changed, and E (electric field) changes according to .

Here is an awesome example of the Theremin in action:

I feel like it sounds similar to a soprano when in its higher register and a cello in its lower register. What do you guys think?

Progress on my app progressed smoothly last week, and as of last night, APlusPhysics is an official app on the Android™ market! The app can be found at:

Androidzoom.com, Appbrain.com, or Androlib.com.

Alternatively, one of these QR codes can be used to access the app's page (they are the same besides their size):

[ATTACH=CONFIG]74[/ATTACH][ATTACH=CONFIG]75[/ATTACH]

Enjoy!

Android™ is a trademark of Google Inc. Use of this trademark is subject to Google Permissions.

My recent coding endeavors have been channeled towards the development of an Android™ application for aplusphysics.com. I intend to create an app that serves as a smooth interface with the most important information present on aplusphysics.com and allows the user to experience the content quickly. While many visitors of aplusphysics.com are part of our physics class, I intend to make this app useful to anyone around the world that desires to learn more about physics.

So far, the app shows blog updates and podcast updates, and allows the user to download or listen to audio podcasts. Here is a picture of the home screen so far:

[ATTACH=CONFIG]73[/ATTACH].

As you guys can see, there is room for another button on the right. I still haven't decided what I want to put there, but if anyone has an idea for an additional feature of integration with aplusphysics.com, I'd be happy to hear it!

Android™ is a trademark of Google Inc. Use of this trademark is subject to Google Permissions.

Our AP-C class just had our multiple-choice/FRQ unit test on rotational motion, and with 20:20 hindsight I have a few tips for anyone looking to tackle his or her exam.

I. Know your formulas.

Make sure that you not only know the syntax of each formula, but also the meaning behind it. Formulas are always a representation of a relationship between two or more variables, and sometimes they will be tested in this specific manner. For example, with the formula Force(centripetal) = (mv^2)/r, the formula itself is important, but the fact that an increase in radius decreases the amount of centripetal force is equally as important.

II. Remember what you have, and use it to find what you need.

Many complex FRQs, and some multiple choice, cannot be solved with the use of only one formula. Sometimes one simple conversion is required to bridge the gap between two realms of givens, especially in rotational motion. Remember that all of the kinematics formulas can be converted into their rotational equivalents. An example of "bridging the gap" is found in a problem where two blocks are hung over opposite sides of a pulley with non-negligible mass. First, the net force in both the x- and y-directions must be found. Then the rotational motion of the pulley must be accounted for in a new torque calculation. Finally, the angular acceleration must be converted to translational acceleration to relate the two types of motion.

III. Use units to your advantage.

Before panicking over a confusing multiple choice question, remember that there is more to an answer than the number. If the units differ between answers, attempt to find what the units should be using your memorized formulas.

Check this out (at 4 minutes, 10 seconds):

*Please use this link to watch the video at the appropriate time:

When I first watched this clip, I couldn't believe it... After all, we have always been taught that an object in motion stays in motion unless acted upon by an external force, and I saw no strings attached to Professor Bowley.

As Bowley explains, the phenomenon of his rotations are caused by conservation of angular momentum. We all know and love the conservation of linear momentum (), and the conservation of angular momentum can easily be derived by using the translational-to-rotational substitutions found in many of our rotational motion blog posts.

Conservation of angular momentum:

This equation explains to us why a changing of the direction of the wheel's angular momentum causes a change in the professor's angular momentum. When the wheel is flipped, its angular velocity (and hence angular momentum) is negated. In order to compensate for the change of the wheel's angular momentum, the professor's moment of inertia or angular velocity must change. In this case, he keeps the wheel the same distance from his axis of rotation (keeping his moment of inertia constant) and ends up increasing his angular velocity, causing him to spin.

First, here is some Swiss ski flying: (please note: for the purpose of this post, the mentioned video is not used for educational purposes, but instead serves as a really cool sight that is hopefully inspirational to readers)

http://www.youtube.com/watch?v=utMUWpF_xC0

And now for your weekly dose of physics...

Sometimes one may feel like he or she cannot count on anything, like life has no constants and is unbearably unpredictable. But humans have always taken one constant for granted: a new day is always coming. The phenomena called the "day" is the unit of time measuring how long it takes for our earth to rotate one complete revolution. When earth rotates, the sun's light hits a new section of the earth, giving us sunrise and sunset. Everything from the workday, agriculture, visibility, human instinct, and Swiss ski flyers rely on the presence of the sun to function properly.

Okay, so we get that the day is important. But what if this thing that we always take for granted was not actually inevitably reliable? That's right, the day as we know it is coming to an end. More on this later.

First of all, taking the earth out of its orbital context, the earth's rotational kinetic energy is defined by

is the moment of inertia (used similar to mass in translational motion). The general definition of moment of inertia is .

Assuming the earth is a solid sphere (and in this case, of uniform mass), it's moment of inertia would be . With a mass of ~ (thank you google.com) and a radius of ~ (google.com, what don't you know?), earth must have a moment of inertia of

(deceivingly pronounced "oh-may-guh" and not "double-you") is the angular velocity. is similar to translational velocity, but instead it tracks the speed of particles as a radians/second measure around a circle. According to http://hypertextbook.com/facts/2002/JasonAtkins.shtml, the angular speed of the earth is ~

With this information, we can calculate earth's kinetic rotational energy.

That's a lot of Joules!

In a perfect world, this energy will not change and our days will remain intact. Unfortunately, perfect this world is not. What could change our lovely earth's rotational speed? One may first answer force, but in the rotational world of physics, we convert forces that cause rotation into torque (or ).

This means that with a net force of 0, no net torque would be exerted on our planet and the day would remain constant. In reality's case, the friction of the tides being pulled by the moon is causing the day to shorten by 2 milliseconds each year. The question is: what torque are those pesky tides exerting on our beloved home? Let's find out!

For this we will need a couple more rotational formulas. Note these formulas can be derived from kinematics equations using m = I, X = , v = , and a = .

Finding the change in :

Finding :

Finding torque!

In retrospect, that is a lot of torque, even when considering that our earth is ~75% water on the surface.

Copyright Notice: I do not own any portion of this video. All credit goes to the posters of it on Youtube.com and to the owners of each clip.

Here's a question... how can one pool ball knock six others into the pocket? The answer: conservation of momentum. I like to think of the stagnant six pool balls as one single object. If the cue has mass 'M,' the object including the corner balls has mass 6M. Using conservation of momentum... [ATTACH]50[/ATTACH]

and that after this (nearly) elastic collision, the velocity of the cue ball is 0...

MV + 6M(0) = M(0) + 6MV'

Therefore, V', or the velocity of the six balls, is V/6.

The smaller velocity of each of the six balls explains why one ball can move six. While each ball after the collision is moving slower, the sum of all of their momenta equals that of the cue ball before the collision.