“Mad Science” Mini-Lessons

5. Waves in a Rubber Band

Topic

The nature of waves; wavelength, frequency and amplitude; wave speed; polarization; superposition; reflection; quantization of energy levels

Supplies

rubber cords, both the stretchy and the taught kind, one per student group, two stretchy cords if they are shorter than 2m or if you just want more fun; rigid rod, such as a wooden stick; length of heavy thread, string, or light cord; tie-down location for each student group (window bars work well).  For extension: Slinky

Procedure

Tie one end of a stretchy cord and hold the other end under tension in your hand.  Strike the cord near your hand.  Or, “pluck” the spring by holding the end fixed and lifting a nearby point, ( band pulled sideways near one end) then letting go.  What happens?

Strike or pluck the cord in different directions: from above, from beneath, from the left, and from the right.  What is the wave pulse like in these situations?

What happens to a pulse when it reaches the fixed end? 

Stand farther away so that there is more tension on the spring.  Again, strike or pluck the cord near your hand.  Is there any difference from the previous times?

Move the free end up and down, slowly at first, and then faster.  With a little practice, you should be able to produce patterns known as standing waves.  Although the cord moves up and down, the waves stay in place.  The places where the cord does not move are called nodes; the places where the cord stays in place are antinodes.  You should be able to make different patterns.  How many antinodes can you make in a standing wave?

How are the frequencies of the standing waves related to their wavelengths?  Is there a way you can measure the frequencies and wavelengths?

Remain where you are and change the tension in the spring or cord by feeding or taking up some slack.  Create the same standing wave patterns as before.  How do the frequencies compare to the frequencies of the same patterns under different tension?

Can you also make standing waves by moving the spring side-to-side rather than up and down?

Untie the cord from the tie-down and tie it to a length of light string.  Tie the other end of the string to the tie-down.  When you make a wave pulse in the cord, how does the pulse behave when it reaches the string?

Repeat with a cord that does not stretch so much under tension.  How do waves behave in it?

Extension

With a Slinky, it is possible to show longitudinal as well as transverse waves.  A slinky sags a lot under gravity, so it is necessary to rest it on a level surface, such as a table or the floor.  Be aware that friction absorbs a lot of the energy in that circumstance.

Questions to consider


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Revised: 21 December 2016; Maintained by Richard Barrans.
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