Comments/Suggestions From the Net (Thank you!)

Your Suggestions/Comments are highly appreciated! Send me Mail


Subject: Buoyance force
Date: Sat, 16 Aug 1997 07:09:52 -0500
From: "K. M. Isaac" <>
Organization: MM Enterprise
Mr. Hwang,
Your buyancy force animation is very good. I plan to use it in my fluid mechancis course.
K. M. Isaac University of Missouri-Rolla

Subject: Greeting
Date: Wed, 20 Aug 1997 16:29:26 -0500
From: "efra璯 barbosa" <>
Organization: Yoshida-Barbosa
Dear Prof. Hwuang:
I am also a physicist. I live in Bogota, Colombia.
I enjoy your simulations very much. It was a great work done by you. Congratulations!
Efraim Barbosa

Subject: WOW! Great sight
Date: Fri, 29 Aug 1997 16:48:04 -0600
From: Gary S Smith <>
John P. Cise..Fac. Austin Com. College

Subject: EM wave Applet
Date: Mon, 01 Sep 1997 17:34:13 -0700
From: "Sunil K. David" <>
Organization: Bethune-Cookman College
Hi: You have made my job quite simple in showing the prop. of EM wave for my class.
Congratulations. Great applet. Goodluck, Bye,
Sunil K. David, M.Sc., M.Sc. Ph.D.
Assistant Professor of Physics

Subject: Your Physics Web site
Date: Wed, 03 Sep 1997 16:20:01 -0500
From: Narsing Rao <>
Organization: Bethune-Cookman college
Hello Dr. Wang:
    I just browsed your physics web site. I am really impressed by what I saw.
I am sure my students will find it very useful and exciting.
I am not a programmer either, and just now I am trying to learn about HTML and JAVA etc.
So long. I 'll be watching more of your pages.

Subject: congratulations
Date: Sat, 06 Sep 1997 14:18:50 +0800
From: Cho-Teng Liu <>
Dear Prof. Huang:
Your web pages are very exciting and it deserves an award in education community.
A minor suggestion:
While watching some of the experiment, e.g. the RC充/放電 電路,
the browsers will be more comfortable if they can view both the figure and the text on the screen.
Therefore, I would like to suggest that
1) add a scroll bar for the text, or
2) place the title and figure on the left frame, and the text on the right frame.  
Congratulation to your accomplichment.

Subject: java and sound
Date: Fri, 12 Sep 1997 14:19:41 +0800
From: "Harlan W. Lefevre" <>
Dear Dr. Hwang,
I have used several of your applets in my physics courses here.
And I asked our people to mirror your site here for fast access.
The applets are really useful. Thank you.
I usually roll an NCD Xterminal into my classroom and use a remote Sun as server.
Unfortunately java only writes audio to /dev/audio and Xwindows won't deliver the audio to the Xterm.
So the beauty of your Fourier Synthesis program is largely lost when one can't do the sound.
Do you have a solution to this problem?
Sincerely yours,
Harlan Lefevre

Subject: pulley applet
Date: Fri, 26 Sep 1997 09:21:00 -0400
From: "Robert B. Muir" <>
Organization: Physics & Astronomy, UNC Greensboro
You have done a very good job developing these wonderful applets. 
I have not run all of them yet, but look forward to doing so.
I just tried the Pulley applet and had a problem. 
It seemed to download, but nothing ever appeared in the display area. 
The text below the display area looked ok. 
I am running Netscape Communicator 4.01 on a 166 MHz Pentium with 30 Mb of Ram. 
All the other applets I have tried worked ok.
Thank you for making this material available to us all.
Sincerely yours, Bob Muir
Physics & Astronomy
U North Carolina at Greensboro
Greensboro, NC USA

Subject: Hallo
Date: Tue, 7 Oct 1997 00:29:04 +2.00
From: "H L Botha" <>
Organization: Iscor Profile Products
Hallo Mr Hwang
I enjoyed your web site , good work !!
Herman Botha (South Africa)

Subject: hotjava!
Date: Fri, 17 Oct 1997 13:19:20 +0000
From: David Johnson <>
Organization: Douglas High School Science Department
hello Fu-Kwun
I've used your vector addition and vernier caliper applets in my physics and physical sciences classes.
I'm just sending this note to thank you and ask you to keep up the good work!

Subject: your educational Java applet(s)
Date: Mon, 20 Oct 97 17:53:34 -0700
From: Martin Koning Bastiaan <>
To: "Fu-Kwun Hwang" <>
Dear Java Educational Applet Author,
Thanks for creating a great educational applet and making it available on the WWW.
Your Java applet is pointed to by the Educational Object Economy (EOE), located at
Apple Computer, Inc.零 EOE Team is preparing to distribute an EOE framework
(which we雹e calling the GOE--Generic Object Economy)
and its URL directory to groups who wish to set up their own EOE.
Please reply to this message if you would NOT like a pointer to your applet(s) (shown below) included in this release.
If you have written other educational applets in Java and would like to see them included in our EOE database,
please contact us. You can even enter your applets into our database yourself,
If you find any out-of-date or incorrect information, either listed below or on our site
(which contains more thorough descriptions of the Java applets than is listed here),
please let us know so that we may update our records.
For more information about the GOE:
Martin Koning Bastiaan,
Educational Object Economy

Date: Sun, 26 Oct 1997 08:49:37 -0800
From: Ursula Sexton <>
Hello, Mr. Hwang, I am an elementary science teacher,
as well as a leader in science education reform in the state of California.
I train teachers and while searching for some physics principles, I found the Web site with your applets.
I was interested in seeing your rainbow demonstration. It is so wonderful!
This site is just full of great demonstrations for clarifying principles that teachers just can't seem to grasp,
unless a visualization tool is provided. This is just great!
I thought I'd mention that there is a small mispelling at the bottom of your rainbow page.
Your last bullet reads: "Did yo noticed that rainbow consists of partially polarized light.
It should say - for grammar and spelling to be correct:
Did you notice that the rainbow consists of partially polarized light?
Best of luck in your journey to share with others the wonders of science.
Thanks for the gift of your knowledge and the wonderful medium to share it!
U Sexton

Subject: standing wave
Date: Wed, 03 Dec 1997 10:26:03 +0000
From: Keli Sato <>
Organization: Addison Wesley
Hello, I visited your Superposition Principle of Waves (
The applet is great, but I think you ment to say Standing wave, not Standarding wave
Keli Sato

Subject: Superposition of waves
Date: Mon, 15 Dec 1997 20:37:48 +0100
From: Peter Ryder <>
Organization: University of Bremen
Your Java applet for superimposing wave is very nice,
but the "medium" has the interesting property that the group velocity and the phase velocity have opposite signs!
Well, that makes an intersting problem for the students to discuss.

Subject: RC circuit applet
Date: Sun, 11 Jan 1998 06:41:16 +0000 (!!!First Boot!!!)
From: "Samara, Thomas Nick" <>
Dear Author,
Thanks alot for your program.
I couldn't quite find in the book a description of what the current was doing while the capacitor was charging.
Please make more programs like it.

Subject: Doppler effect and shock wave
Date: Sat, 17 Jan 1998 20:49:55 +0700
From: Amrat Chawla <>
Dear Sir,
Even though I am in trading business, in my personal time I am very much interested in Physics
and I bought some 1st year university text book to study Physics as hobby.
But frankly, your web site give me a very clear lesson just in few days
that I discovered your site I understand a lot !!.
For example I try to understand the sonic-boom but even I read many time I still cant get it,
but with just few minute of virtual experiment with 'Doppler effect and shock wave' it became clear to me !!
Really thankyou very much for your site.
I regard you as my teacher.
With respect and best rgds.
Amrat Chawla

Subject: This site is so cool!!!
Date: Fri, 23 Jan 1998 16:10:28 -0600
From: Michele Gale-Sinex <>
Thanks for sharing it.
michele gale-sinex
center for integrated ag systems

Subject: Pulley Experiment
Date: Sun, 25 Jan 1998 13:14:53 -0600
From: "Radim Kolarsky" <>
To: <>
Dear Dr. Hwang:
My son and I have enjoyed visiting your site today and yesterday, looking at your experiments.
In an effort to assembe information for my son's school science project,
we attempted to see the "Pulley" page but, unfortunately, the link is broken.
I have marked your site among my bookmarks and will definitely come visit again.
Radim A. Kolarsky

Subject: Pulley and more...
Date: Mon, 26 Jan 1998 20:57:05 -0600
From: "Radim Kolarsky" <>
To: <>
Dear Dr. Hwang:
Thanks for fixing the link and making the page available again.
I have been looking for a while for this type of an on-line physics textbook. Your page is just great.
Please keep up this good work. I work as a geologist/applied geophysicist at Shell in New Orleans.
Originally, however I was born in Czechoslovakia and came to the U.S as a political refugee in 1987.
I will never forget my high school physics professor (whose picture my wife and I keep on our web page)
I had there as a student. He did quite a bit for me.
Thanks again. SHould you efer come to New Orleans (a convention perhaps) please do not hesitate to let us know.
Radim Kolarsky

Subject: Request for the secret!
Date: Tue, 27 Jan 1998 23:57:11 +0800
From: "Stephen, W. T. Kwan" <>
Dear Sir,
I am Stephen W. T. Kwan, a secondary teacher in Hong Kong.
Your java applets are too amazing that I cannot help writing to you immediately.
Would you please mind telling me how you develop these kinds of programs and
the time needed to do so?
Actually, I want to develop similiar programs myself in Hong Kong.
You can contact me through my e-mail address
Thank you for your attention.
Stephen, W. T. Kwan 27/01/98

Subject: Orbit applet
Date: Fri, 30 Jan 1998 08:44:52 +0530
From: "Surendranath Reddy.B." <>
To: "Hwang" <>
Dear Mr.Hwang,
I just saw your orbit applet. It was just the thing that I thought I would suggest to you.
Please consider the following suggestions.
Would it possible to show the initial position of the projectile,
its velocity and the angle the velocity makes with the line joining it and the center of the earth
and then give the option to launch.
The distance from the center of the earth could be made large ( twice, thrice.. the radius of the earth)
It would be better if the user gives these inputs by mouse clicks or by entering the values.
( It may be also a good idea to relate these values with (gr)^0.5,
the value required for a circular orbit. One could bring out the concept of escape velocity also.
Warm Regards
Surendranath Reddy.B.

Subject: Re:Suggestions
Date: Sat, 31 Jan 1998 11:41:23 -0500
From: "Pierce C. Barnard" <>
Organization: myself
Hi, I was examining your satellite & projectile motion applet and I found it to be very useful.
A suggestion, you may want to allow a user to set all paramters and
provide a button on the applet to start the applet.
This way the user will not have to keep on readjusting the applet while the projectile is in motion.
Pierce Barnard

ubject: Lenses
Date: Sun, 01 Feb 1998 17:08:13 -0600
From: John Eagle <>
I think this page is great.
I stumbled into it one night while trying to find some insight on a physics assignment that I had.
I've used it ever since to check my work.
Something that might be helpful to a great number of people
would be a java applet with thick lenses and or two lense systems. Just a thought.
Thank you for taking the time to publish such a page John Eagle

*** Please check out Thick Lenses java applet

Subject: GO!GO!GO!
Date: Wed, 11 Feb 1998 21:51:07 +0800
From: tiffany cheng <>
hi, This is my first time to be here, your homepage. What can I say???
It's unbelievable! It's fantastic! It's perfect! Here, I'm writing this letter is to encourage you to keep doing this job.
It really gives people answers. GO! GO! GO!

Subject: Your Java Frictionnal Force applet.
Date: Mon, 9 Feb 1998 11:45:31 +0100
From: "leflocht" <>
To: <>
Thanks a lot for your on-line physics lesson.
As a french teacher i would like to ask you the permission to use your applet on a new html sheet
using french as basic language. Of course i would only translate your words( the best i can)
and leave each reference (native URL, mailto.. and comments in original state.)
The reason is that i just gave my students, (aged 17) a exam using the study of this system.
So i would like them to use your applet to get more of this study case by using new parameters
such as Mu and various masses.
This page, if possible, will be added to the site i create for my sudents (it's the very beginning),
you could surf at: and find it using french "Fichiers de TP" in the 1S section.
---- You ask for suggestions ? Of course i've got some.
*first: The masses you use : Are F1 = 10 grams or F1 = 10*g where 10 is kg and g is 9.81 mS-2 ?
(and so on for others )? case 10 grams: we use to calculate Force intensity in Newton (France) case 10 kg:
the mass seems very heavy !
*second: Some lines of your HTML page are not readable for french users because of the asian keyboard
translation,( i think.) Just above the definition table of forces, there is a line i'dont know what it is ?
(caracters like @#& and others not in use in our alphabet but for bad words ...)
The definition for f12 is quite the same it appears like: f12j[number zero]m1 g[symbol for english pound] g
I think we must read: f12 not so different of m1. Mu.g. ad so on for each reading of friction force. -----
I can make, if you look for it, other text translations, so your work to make your work used by french people.
Thanks .
Thierry Le Floch Thanks to disregard my english mistakes.
Thierry Le Floch
Email: site
UIN 4557973

Subject: animations
Date: Wed, 11 Feb 1998 11:43:06 -0400
From: Hugo Beltrami <>
To: CC:
Hello, I was looking at your site and I was wondering whether it would be possible
to mirror it here at this university. Would you please let me know if this is possible.
Then I would contact our system manager Thank you for your attention
-- Hugo Beltrami
Assistant Professor
Voice: 902-867-2326
Department of Geology
Fax : 902-867-2457
St. Francis Xavier
P.O. Box 5000, Antigonish
Nova Scotia, Canada,B2G 2W5

Subject: Applets
Date: Thu, 12 Feb 1998 16:57:50 -0600
From: Taha Mzoughi <mzoughi@Ra.MsState.Edu>
Organization: Mississippi State University
Dear Professor Fu-Kwun Hwang:
This is the first of two emails I will be sending you.
I have been familiar with your work for about a year now. To say the list, it is impressive.
I am an assistant professor of physics at MS State University and I try to use the Web for instruction.
I am not a Java programmer. I wrote some stuff with JavaScript.
The URL for my class is:
I am still working on the content and would like to eventually include some of your Java applets.
Obviously maintaining links to your Web site and proper credit.
I have noticed that you have your applets mirrored throughout the word (which is nice)
and I am wondering if it acceptable for you to add another mirror in our server.
I am also interested in having a copy of the applets to run of laptop that is not connected to the Web.
Actually, I am giving a talk about resources on the Web for teaching physics to high school teachers
on the 20 th of February, and I hope to be able to include some of your applets.
I will write a page with links to the applet in your site that I will use in the presentation then place it on our server.
I appreciate your help. You are doing some neat stuff there.
Taha Mzoughi
Assistant Professor
Department of Physics and Astronomy
PO Box 5167
Mississippi State University
Mississippi State, Mississippi 39762-5167
Phone: (601)325 2923
Fax: (601)325 8898

Subject: Java and Physics
Date: Sun, 15 Feb 1998 09:15:22 +0100
I am a professor of physics at the Royal Institute of Technologu in StockholmSweden.
We do teach Java for programming in Science and I came across your web pages.
Although we do teach Java for Ph.D. students I think there is something that could be learnt
from your beautiful pages. Maybe we will also extend the course to MS levels.
The problem is that we are in physics and Java should be taught by the CS dept, but will they do it our way?
We may try to suggest a course and then some of your web pages could be very useful to the students.
I would therefore like to ask you about your plans etc.
I would hate to have made refs to your pages and then they would disappear one day, smile.
Also, you may have some new ones in mind?
I think something with the uncertainty principle would be very useful. Have you ever considered this?
I am very open for any form of collaboration on this future teaching project.
You can find examples of the present course on the web.
We plan some lecture notes and I hope I can make ref/use some of your examples?
Looking forward hearing from you
// Thomas Lindblad

Subject: Physics Java Applets
Date: Sun, 15 Feb 1998 15:37:15 +0800
From: Stephen Chin <>
Dear Sir, I'm also a Physics teacher, O level, and I am interested and fascinated
by the Java Applets you have created..... they are really good.....
I find them very useful for teaching and demonstrating certain concepts in physics....
I am wondering how do you write a Java Applet....
I have the Java Programmer Kit downloaded from Sun.... but don;t know how to use it.....
please advise
Thanks in advance.
Stephen Chin
-- If we wait long enough we may reap the best harvest....

Subject: Publishing Your Applets
Date: Tue, 17 Feb 1998 18:38:33 -0600
From: Taha Mzoughi <>
To: "''"

Subject: java applets
Date: Sun, 01 Mar 1998 01:45:54 -0500
From: Clifford Sampson <>
Dear Mr Hwang
I have found your java applets especially those on waves and optics to be particularly facinating and informative.
So much so that I have begun to use them extensively for my grade 11 physics students
at Appleby College in Oakville, Ontario, Canada. ...

Date: Mon, 02 Mar 1998 13:32:38 +1100
From: masukor <>
Hi, been to your physics lab. great stuff.
I teach physics at a junior college here in australia.
I certainly will make more visit to your site. bye.

Subject: Your Doppler effect applet
Date: Sun, 8 Mar 1998 22:39:05 -0600
From: "Michael Vershinin" <>
To: <>
Your applet is pretty neat, but since you asked for suggestions...
1) I would not allow the user to set a negative wavelength.
2) If negative source speed is chosen, your source runs off the edge of the window.
3) Negative wave speed is a bit awkward, since your source is actually a sink.
Michael Vershinin

Subject: Thank you for the applet!
Date: Tue, 10 Mar 1998 10:34:26 +0100
From: Uwe Gaertitz <>
To: "''" <>
your applet works fine, thanks a lot.
We want to use it offline, for non-commercial demonstration.
I've mentioned you and made references to your site at this local page, however.
Some suggestions/questions:
1. One goal is to maximize/minimize :-)
the flow. But it's so easy, with setting velocity and acceleration to max(min), isn't it?
2. It would be fine to see the flow rate in an extra text field,
cause this is the much interesting output (for me).
3. Perhaps, a "history" of the last flow rates (then you can better see,
whether you have success in maximizing flow rate or not).
Could be a simple text table (over a time intervall), or a graph.
Ciao, Uwe

Subject: I saw a Polarized Rainbow!
Date: Sat, 14 Mar 1998 01:18:24 -0800
From: Neal Rasmussen <>
I am, as far as I know, the first to see a "Polarized Rainbow"!
I think we should name this phenomona the "Rasmussen-Wang Rainbow". :-)
I am a tornado chaser for 20 years. About 4 years ago I was driving through a McDonald's drive thru.
As it was just after a pretty good storm, I had mys polarized yellow tinted flip-up sunglasses on.
Low and behold I almost dropped my cigarette into my lap.
There off in the east was a double rainbow!
Not the kind I've seen dozens of times where the second one is reveresed and much outside the primary,
but this was immediatly INSIDE the primary!
Gaping with my mouth open and people behind me honking,
I flipped my flip-ups up and the second inner one went away!
I must have sat there for 2 minutes flipping my sunglasses.
The colors were NOT reversed and it looked like one continuous double wide rainbow,
with two complete sets of colors.
I ruled out that the flip-ups where giving double vision as I looked at lights the next day,
streetlights headlights, etc. and no doubles!
Can this possibly be? Could a source of polarized light be at a different angle,
perhaps the sun shining through a cirrus cloud or noctelucent (sp).
Neal Rasmussen

Subject: Can I add a counter to the mirror site?
Date: Tue, 24 Mar 1998 19:02:33 +0800
From: "tanghb" <>
To: <>
Dear Mr. Hwang,
When I meet the physics teachers of the other schools in Singapore,
they give good comments about the mirror site of Virtual Physics Laboratory.
I think your Java Applets are popular in Singapore.
I am therefore interested to know how many people come to visit Virtual Physics Laboratory
and use your Java applets everyday.
Mr. Hwang, can I add a counter on the contents page?
If so, please let me know.
Thank you.
Tang Hung Bun
Singapore Chinese Girls' School

Subject: Lens/mirror
Date: Fri, 03 Apr 1998 16:16:37 -0600
From: Kelly <>
Mr. Hwang, I want to say that your virtual lab was unbelievably helpful to me.
I attend the university of wisconsin-madison and I was having problems with optics concepts.
I went under the internet and searched under lens and mirrors and fell upon your lab.
I am very grateful. I was a little upset last night when I wanted to access the lens mirror applet,
because I couldn't find it.
I guess even the virtual labs have hours of availability.
I guess that's my only suggestion,
which isn't a very necessary one to attend to when most of us can access it during the day.
Thanks again, Chris

Subject: Refraction Applet
Date: Thu, 23 Apr 1998 10:14:02 -0400
To: hwang <>
Is there any way to change the velocity vector and/or the medium thickness
on your refraction applet if you don't have a two button mouse?
Sandee (Lakota East High School Physics)
P.S. -- you have an incredible site

Subject: Java 動畫
Date: Mon, 27 Apr 1998 18:44:50 -0700 (PDT)
From: Dan Murray <>
黃教授﹐你好﹗ 新近﹐我尋找了網際網絡﹐找了有意思有關物理的事﹐
也發現了國立 台灣師範大學的物理系的網頁﹐所以發現了你的許多 Java 動畫。
直到 現在﹐我沒有意識了物理教師創造了那麼許多 Java 動畫。
但是﹐你供 應更多消息是適當。我的電腦是 Windows 3.1 的 PC 。
即使我使用比 較新近的 Netscape (就是 3.05)還不可以看見 Java 動畫。
最低的 電腦需要是 Windows 95。或許你可以給人講解﹐所以阻止挫敗。
我們 有 Windows 3.1 的人應該知道﹐ 惟有看見 Java 動畫的方法大概是買 新電腦。
可惜﹐因為我是物理教授﹐無奈繼續使用我的舊電腦﹗ Dan Murray 謹上

Subject: (no subject)
Date: Wed, 29 Apr 1998 22:13:08 -0500
From: Dion Sanchez <>
I want to thank you for this page, it has helped me visualize what a text book could not.
I have been looking for something like this for a long time.
great stuff thanks. dion

Subject: Wow!!!
Date: Sun, 03 May 1998 12:47:55 -0600
From: The Lahrs <>
I love your thin lens page:
What a great use of Java!
I use Netscape, by the way, and the applet worked just fine and was very speedy.
I've been trying to figure out why a glass rod creates an inverted (about one axis) virtual image.
Do you have an applet for a thick lens, or just a ray diagram that would show this effect?
Is the code still available? The link was not working when I tried.
It would be interesting to develop something like this for seismic waves.
I'm a seismologist now, but my first love is physics,
which was my undergraduate major at Rensselaer Polytechnic Institute.
Thanks again for providing the world this great applet.
John C. Lahr
-- * John C. and Jan H. Lahr * * * *
914 10th Street * *
Golden, Colorado 80401 * *
(303) 215-9913 * * * * *
Subject: Wow!!!
Date: Sun, 03 May 1998 15:39:11 -0600
From: The Lahrs <>
I just figured out that the inverted image is not virtual but real,
so everything makes sense now.
Keep up the good work.
John C. Lahr

Subject: problems with cyclotron java applet
Date: Thu, 4 Jun 1998 19:37:38 +0200
From: "Pietro Diviacco" <>
To: <>
My name is Pietro Diviacco. I am an Italian surgeon and live and work in Genova, Italy.
I tried to play Your applet "Cyclotron" but there was some problems.
Some of the images (*.gif 7 and 14) were not available and the applet was not working.
In any case Your applets are very good and interesting (the working ones, of course).
Thank You in advance for fixing the problems.
E-mail address:

Subject: Web Site
Date: Mon, 8 Jun 1998 18:11:12 EDT
Hi! I'm a physics teacher in Blantyre High School in Scotland
and I've just discovered your Physics demos site.
When I return to school tomorrow I'll be directing my class to your site first thing.
Well done and thank you...
I'll be a regular visitor from now on.

Subject: Great jobs!
Date: Sun, 14 Jun 1998 17:37:43 +0800
From: "Robinson Wang" <>
To: <>
Dear Mr. Hwang,
Hello! I am a student of the National Tsing Hua University(NTHU).
Maybe you've heard that the Material Science Engineering which is my major.
I'd like to appreciate the great jobs you've done.
At first, I come to this website because of my final-term physical report.
However, when I view the first exam, I can't help finishing all items in this site.
Well, that's all what I want to say about this special work.
Have you a happy time in teaching.
Your sincerely, Robinson Wang '98.6.14

Subject: Enjoyed your supersonic applet
Date: Mon, 29 Jun 1998 15:30:54 EDT
I enjoyed studying your applet.
I have taken the liberty of making some minor wording changes so that your description will be more grammatically correct.
I wish I were multilingual as you are! ;-)
The location of supersonic airplane.
If an airplane is flying faster than the speed of sound, it will produce a sonic boom or shock wave.
As shown in the right figure, consider an airplane which is flying from A toward B.
A listener is located at C. The sound generated at A travels toward C along the path AC.
The listener hears the sound as the airplane flys toward point B.
( AB > AC ) DC is the path of the sound generated at D to the listener.
The airplane speed is V and the sound speed is Vs.
If AC/Vs > AD/V + DC/Vs [Isn't it unlikely that the shorter path will take a longer time?],
then the sound generated at point D will arrive earlier than sound generated at point A! Funny!
You hear something which happened later and then hear something which happened earlier !
What will you hear if a supersonic airplane flying over you?
Do you know where is the airplane when you hear it?
Can you point out the direction of the airplane just from hearing the sound?
This java applet let you play with such situation.
Press Start to start the animation. A listener is represented by an ear!
You can press the left mouse button and drag it within the screen to change location of the listener.
The program draws sound wave paths to the listener.
The animation will be suspended when the sonic boom arrives at the listener.
Click the right mouse button to continue. The color of the paths changes to blue.
The order in which the sound paths arrive at the listener are shown also.
You can change the ratio of airplane speed of airplane to sound speed and see the difference.
John Rice

Subject: Interesting
Date: Sat, 04 Jul 1998 11:57:20 -0700
From: Daniel Murphy <>
I'm a student at Olympic College in Bremerton, Washington currently enrolled in a physics class.
We are studying vectors, freefall, velocity/acceleration and projectile motion.
Your web page is great!
I think the answer to the question of case one about the green circle is the arc of zero vertical velocity.
When the object reaches zero velocity, acceleration remains a constant 9.8 m/s(sq) downward.

ubject: Your applets
Date: Fri, 24 Jul 1998 13:13:16 -0400
From: (Mike Diegel)
My friend and I had been discussing what a sonic boom is
and I was trying to describe how the sound waves come together to make it happen.
We came across your web page at
and used the applet. We thought it was a fantastic illustration of what a sonic boom is,
and we just wanted to let you know that your work was appreciated.
Thanks for the help.
Mike Diegel 966-9100

Subject: Conservation of Linear Momentum
Date: Tue, 28 Jul 1998 09:14:28 +0800
From: Lim Ai Phing <>
I'm a physics teacher presently teaching my students dynamics.
Your collision java applet is really good, I'll ask my students to take a look here!!
But can I suggest that the colour of the numbers be changed
because yellow doesn't show up much against a gray background?

Subject: Thanks for great BioSavart
Date: Mon, 24 Aug 1998 22:16:57 -0700
From: "Sunpainter" <>
To: <>
I was looking for something on the shape of circuit for very small circuit
trying to predict some electromagnetic field changes in the circuit.
I will be using very, very small current.
Your simulation was incredibly, wonderfully helpful!
If I had to do all the math for each current change, it would have taken me forever.
Now I have my figures. THANK YOU THANK YOU!
It is for a strange little generator I am building.
Thank you!
K. Hodge Southern California

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作者:國立台灣師範大學 物理系 黃福坤