Reflected Pulses on a Spring

Purpose:
To demonstrate how a pulse is reflected both when there is a fixed end and a loose end. This is also demonstrating how the tension in the spring affects the speed of the waves.

Equipment:
Long spring with string attached

Equipment Location:
Other

Procedure:
For the first part of this demonstration have two people hold the ends of the spring. Have one person strike the bottom of the spring while the other tries to keep their end of the spring still. A wave should travel down the spring above where the spring was originally and return below where the spring originally was when the wave was started. For the second part of this demonstration have one person hold the end of the spring and the other hold onto a string that is attached to the other end of the spring. Have the person who is holding onto the spring make the same striking motion as before. At the same time have the person holding onto the string keep his end completely still again. This time the wave should travel down the spring on top of where the spring originally was and also return on the top of where the spring originally was when the wave was started. For the last part of this experiment have a student hold either side of the spring. Have them stand at a distance that allows the spring to still have a small amount of slack. Have one student initiate a few waves down the spring and have the class observe the speed at which the wave travels. Now have the students stretch the spring much tighter and again have the student initiate waves down the spring. The time when the spring is tighter should have waves that travel faster than the one where there was more slack in the spring.

Hints:
It may be necessary to explain that since the string is not completely free that the reflection will not be complete which could make the reflection harder to see. The only other problem with this demonstration is that when it is performed in Masters 208 it may be necessary to have the person holding the string stand out in the hall in order to get the spring as tight as possible. Also the student holding the string should hold the full length of the string or the reflection will not be as complete as desired.