# Introduction to Waves - DOC by liuqingyan

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```									                                         Introduction to Waves
Waves are the transfer of energy created by vibrations.
Mechanical waves require a medium (particles).
Electromagnetic waves do not. They can travel through space (a vacuum)

This introduction focuses on mechanical waves. We will look at electromagnetic waves later.

Behavior of a medium as a wave travels through it.
Click on the following link and experiment with the way molecules of a medium can behave as a wave travels
through it. How does the medium density affect the speed of the wave?
http://sunshine.chpc.utah.edu/labs/waves/wave_basics/waves.htm

Which of the four options, do you think, represents how water molecules behave either in the oceans or as
ripples spread out on a pond.
http://www.forgefx.com/casestudies/prenticehall/ph/waves/waves.htm

SIMULATION ACTIVITY I: Waves on a String
• Load the simulation located at:
• Click on Run Now!
• Modify the java applet as instructed and answer the questions below.

Questions:
1. Wave aspects: Select Pulse mode and click on pulse. Experiment with the 4 different controls.

Effect on wave pulse after varying the following:
Amplitude
Pulse width
Damping
Tension

2. Using pulse mode, set the tension to high and lower the damping to 1.

Describe what happens to single pulse after it reaches the following:
Stiff End
Loose End
No end

3. Colliding pulses: Set the damping to zero. Using a stiff end, send one pulse then send another when the first
one starts to reflect.
What happens when two opposite pulses collide (as in diagram)?
4. Now repeat the above collision experiment using a loose end.
What happens when two pulses on the same side collide?

5. Wave speed: Set tension on high and damping zero. Now click on the timer and ruler and determine the
velocity of a single pulse that travels all the way down and back. Experiment and see which of the 4 wave
controls affects the wave speed.

Effect on wave speed (increase, decrease or no change)
Amplitude
Pulse width
Damping
Tension

Are the waves on the string transverse or longitudinal? Click on the following link if you are unsure about these
terms.
http://hyperphysics.phy-astr.gsu.edu/hbase/sound/tralon.html

Visit the following site to determine how the behavior of a wave on a string changes as it meets a new density.
http://paws.kettering.edu/~drussell/Demos/reflect/reflect.html

Both the reflected wave and transmitted wave (in new density) should be compared to the original incoming
wave.

Low to High Density

Reflected wave      Transmitted Wave
Width (increase or decrease)
Speed (increase or decrease)
Amplitude (increase or decrease)
Orientation (upright or inverted)

High to Low Density

Reflected wave      Transmitted Wave
Width (increase or decrease)
Speed (increase or decrease)
Amplitude (increase or decrease)
Orientation (upright or inverted)
SOUND
Look at the following sites to familiarize yourself with the Physics of sound waves.
http://www.fearofphysics.com/Sound/dist.html

http://www.physicsclassroom.com/Class/sound/u11l1c.cfm

SIMULATION ACTIVITY II: Sound
• Click on Run Now!
• Modify the java applet as instructed and answer the questions below.

1. The simulation should default to “Listen to a single Source”. If not, click on the tab at the top of the page
titled “Listen to a Single Source”. Observe the sound waves coming from the speaker.
a) What do the dark and light bands represent? Remember, sound waves are compression waves.
b) Why do the waves become lighter with distance from the speaker?
c) How does changing the frequency and amplitude affect the depiction of the sound waves in the
simulation?
d) How do you think changing the frequency and amplitude affect the sound heard by the listener?
2 Click the “Measure” tab at the top of the simulation page. Move the ruler to the center of the speaker and
press “start” on the stopwatch.
3. Hit stop and reset, and measure the distance a wave travels in a certain amount of time.
Find the speed of sound using velocity = distance ÷ time (v = d ÷ t).
4. Use the ruler to measure the wavelength (λ) of this sound wave. Make sure you measure the distance
between identical positions on two successive waves. For example, measure from the beginning of one
black band to the beginning of the next black band. Check the speed calculated above using
velocity = frequency × wavelength (v = fλ).

What affects the speed of sound waves?
http://hyperphysics.phy-astr.gsu.edu/hbase/sound/souspe2.html

Doppler Effect