## Illustration 0.1: Static Text Images Versus Physlet Animations

This page describes how to use a Physlet. First of all, a Physlet is a Physics (Java) Applet. You will use the Physlet to view a simulation and answer questions. Sometimes, you will need to collect data from the simulation and perform calculations in order to answer the question. And sometimes, simply viewing the animation will be enough.

Image from Isaac Newton's Principia (1687).

Although the Physlet animations presented in the Workbook will be similar to the static images in your textbook, there are differences that need to be examined, since we will be making extensive use of these types of animations throughout the Workbook.  First consider the above image taken from Sir Isaac Newton's Principia. It is a static image depicting possible orbits around the Earth.  We are supposed to imagine objects thrown from the mountain top with different velocities and image where they would land.  (We are also supposed to imagine that for just the right conditions, objects could orbit in the circles further out from the center of the earth.)

Now consider the Physlet animation of a similar situation.  This Illustration shows 10 identical objects about to be thrown off a mountain top.  The initial positions of the planets are identical but they have a different initial velocity. Restart

To begin the animation press play.  Note that the VCR-type buttons beneath the animation control the animation much like buttons on a VCR, CD, or DVD player.  Specifically:

• play:  starts the animation and continues it until either the animation is over or is stopped.
• pause:  pauses the animation.  Press play to resume the animation.
• <<step:  steps the animation backward in time by one time step (the size of the time step varies with the animation).
• step>>:  steps the animation forward in time by one time step.
• reset:  resets the animation time to the original time.  Press play to start the animation from the beginning.

Make sure you understand what these buttons do since you will need to use them throughout the rest of the Workbook

In addition to these buttons, there are hyperlinks on the page that control which animation is played.  For example, on this page Restart, reinitializes the applet to the way it was when the page was loaded.  On other pages there will often be a choice of which animation to play, but Restart always gets you back to the initial condition the animation was in upon the page loading.

So what is so neat about this animation compared to the static image?  Plenty.  Most of what you will study in Physics is related to objects in motion.  It is difficult to understand the details of the motion of an object if you are trying to describe it by a static picture.  Since the examples in this workbook are animations, you can actually see the details of the motion as the objects are actually undergoing their motions.

Restart (or reset) the above animation and play it again.  What do you notice about the properties of the motion of the balls?  Specifically, what can you say about the motion of the balls that have orbits inside of the red ball?  What can you say about the motion of the balls that have orbits outside of the red ball?  First, all of the orbits are squashed circles (called ellipses) except for the red ball which moves in a circle.  Second, all of the balls---except the red one---change speed throughout their orbits.  The inner balls travel faster near the bottom of the screen as opposed to the top, while the outer ball travel slower at the bottom of the screen as compared to the top.

This is not something that is obvious from Newton's drawing from the Principia, but it is clear from the animation.  This effect is easier to see with only three balls.  Don't forget to press play after selecting the hyperlink!

In the natural sciences simulations are almost always deterministic.  By deterministic, we mean that the simulation evolves in time according to a predefined mathematical model.  The models we have built for this text may or may not represent physical reality.  In fact, we will often present various models and ask you to determine which model is in agreement with experiment.  Do not assume that every simulation obeys the laws of physics.

It is important not to confuse deterministic with predictable.  Simulations that depend on random numbers, contain large numbers of parameters, or exhibit chaos are often not predictable in the sense that the exact behavior may depend on infinitesimal changes of initial conditions.  But, even if the details of the dynamics cannot be determined, the model may still give useful information about the types of behavior that can occur.

Image by Isaac Newton.
Script by Mario Belloni and Wolfgang Christian.
Illustration by Mario Belloni and Wolfgang Christian.
© 2003 by Prentice-Hall, Inc. A Pearson Company