Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne Swinging Sword and Sound
Name: CJ G.
Status: student
Age: 15
Location: N/A
Country: N/A
Date: 2001-2002


Question:
Why does a sword make noise when you swing it through the air?


Replies:
Hi, Christine !!

When a sword moves against the air it compresses the air molecules ahead of it. The air becomes like a compressed coil, that tends to recover its original state. And it does, in fact !! When this happens, other molecules of air - also ahead - become compressed, and tends to recover its original state...and finally this wave reaches our ear. And you can hear the sound.

This happens in all directions, and the sound wave travels even with greater velocity than does the sword. And so, it is possible to hear the sound before the sword reaches the place where it is going to. In the case of the CONCORDE, this airplane is so fast, that it can travel ahead of the wave sound !!! Inside the airplane dominates the silence...

Another interesting point is what happens when a train travels in OUR direction, and the driver sounds the horn. At first, you hear a shrill sound (high frequency) and when it GOES AWAY, the sound is muffled (low frequency). You can understand frequency, in this case, as the number of waves that reach our ear in a lapse of time. This is called DOPPLER EFFECT.

And - last but not least - the same happens with the light !!! In this case, what changes is the colour of the star : shift to red, it the star goes away; shift to the blue, if the star travels with high speed to us !! And Hubble - the astronomer - discovered that the light of the stars tend to ... BLUE, what means, the universe is expanding. Interesting, isnt ??!!

Alcir Grohmann


Without getting into too much of the physics, the sound originates from two sources.

The first is the sword compresses the air in front of it and de-compresses the air behind it. This produces a compressional sound wave. You can do the same thing with a rope or a long stick. The second is that the sword itself acts like a "tuning fork" and vibrates perpendicular to the face of the blade. This sound will depend upon the size and shape of the sword as well as the material from which it is made.

Vince Calder


CJ

The sword smashes into air molecules in its way, pushing them out of the way. When these molecules move back together after the sword moves out of the way, they bang into each other. All this sudden movement causes vibrations of the molecules. These vibrating air molecules make the neighboring molecules vibrate. The vibrations continue on like ripples on water continue if you tap the surface. These "ripples through the air" are what make up sound waves.

If the sword moves slowly, the waves ripples slowly and with little energy. Your ears cannot hear this. As the sword moves faster, the waves ripple fast enough to sound like a low-pitched hum. As the speed of the sword builds up even more the rate of rippling (called frequency) gets high enough to sound like a squeal. Faster swords also produce more volume.

Dr. Ken Mellendorf
Illinois Central College



Click here to return to the Physics Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (help@newton.dep.anl.gov), or at Argonne's Educational Programs

NEWTON AND ASK A SCIENTIST
Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory