PHYS 2425 – Engineering Physics I

Introduction to Waves

 

Leader: _____________________________  Recorder: ___________________________

Skeptic: _____________________________ Encourager: _________________________

 

Materials

Slinky                                                               Metric Tape Measure

Stop Watch                                                      Ring Stand with jaw clamp

Spring Scale (5 N capacity)                              LabPro

ULI Microphone                                              Laptop

2 m cardboard tube                                         

 

Part 1:  Types of Waves and the Definition of a Wave

Stretch the slinky a distance of several meters on the floor.  Shake the slinky sharply to the right or left one time.  In so doing you will produce a wave pulse.

 

1)  Sketch the resulting behavior of the slinky.

 

 

 

2)  Is there a net motion of the slinky?

 

 

The first type of pulse you launched was called a transverse pulse.

3)  Explain the relationship between the direction the wave traveled and the direction of the disturbance in a transverse wave.

 

Sharply push the slinky inwards one time.

 

4)  Sketch the behavior of the slinky.

 

 

 

5)  Is there a net motion of the slinky?

 

The second type of pulse you launched is called a longitudinal pulse.

6)  Explain the relationship between the direction the wave traveled and the direction of the disturbance in a longitudinal wave.

 

 

7)  If there is no net motion of the slinky, what does move when a wave travels along a slinky?  Explain.

 

The slinky is called the medium through which the wave moves, and what moves through the medium is called a disturbance.

 

Place a light object like an empty soda can near one end of the slinky.  Create a transverse wave pulse by sharply shaking the slinky one time to the side. 

 

8)  Were you able to move the light object?  (If not, position the object closer to the slinky and try again.)

 

9)  If the light object was initially at rest, and then began moving it gained kinetic energy.  Where did this energy come from?  Does a wave carry energy?

 

 

With this series of observations, you have seen the basic behavior of all waves.  Since there is not net motion of the slinky, we refer to the slinky as having been disturbed from its rest, or equilibrium position. 

 

10)  Complete the following:  A wave is a motion of a __________ through a ________ which can transport ________________.

 

Now, shake the slinky continuously back and forth.

 

11)  Sketch how the slinky appears.

 

 

When you generate a wave continuously as you just did, it is called a wave train or else a continuous wave.

 

12)  Was the wave train you created transverse or longitudinal?

 

13)  Describe how you can move the slinky so that you produce a longitudinal wave train.

 

 

14)  Carry out the procedure you described in 13) and sketch your results below.

 

 

15)  Sketch a transverse continuous wave in the space below.

 

 

16)  Sketch a longitudinal continuous wave in the space below.

 

 

 

Part 2:  Speed of Waves

      In part 2 of this activity, we will explore the dependence of the speed of a mechanical wave on a slinky on the tension in the slinky and the linear mass density of the slinky.  We will make use of traveling waves like those we made in part 1.  Traveling waves travel down the length of the slinky.  In part 4 of this activity we will investigate another type of wave called a standing wave.

 

First determine the mass of the slinky.

 

1)  m  =

 

      Now stretch the slinky on the floor a distance of 4 m.  Have a person holding the slinky tightly at either end.

 

2)  Record the length of the slinky

 

L =

 

3) What is the mass per length (or the linear mass density) of the slinky?

 

m ==

 

      Launch a single transverse pulse of the slinky and record the round trip time of the pulse.  To obtain better data, repeat the measurement 10 times and find the average of your round trip times.

 

Trial                             Value

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

 

4)  Record the average round trip time

 

t =

 

5)  What was the speed of the wave on the slinky?

 

=

 

6)  Why is there a factor of two in the previous formula?

 

      With the slinky stretched in the same way over the same distance, support one end of the slinky with the spring scale.

 

7)  Record the tension in the slinky.  Remember, the tension is a force, so use the correct scale on the spring scale.

 

T =

The theoretical prediction for the speed of the wave on the slinky is given by

 

8)  Calculate the theoretical prediction for the speed of the wave on the slinky

 

v =

 

9)  How do your values compare?

 

 

Part 3:  The Speed of Sound in Air

      We can use a very similar idea to the one we just used to measure the speed of sound in air.  We will measure the time it takes a sound wave to travel round trip in a tube of air.  Knowing the length of the tube, we can determine the speed of sound in air.

 

Procedure

1.  Set-up

      Measure and record the length of the tube.

 

Tube Length = _____________

 

Clamp the microphone in the jaw clamp.  Connect the microphone to CH1 on the LabPro and connect the LabPro to the computer with the USB cable.  Position the microphone so that it is at the opening on one end of the tube without touching the tube.

 

2.  Start LoggerPro and open the experiment file “Probes & Sensors”=>”Microphone”=>”Microphone.cmbl”.  Click on the data collection button , click on the Triggering tab on the box that opens, and check the box labeled Triggering.  Once you have checked, the Triggering box, change the value of 10 to a value of .05.

 

Zero the microphone by pressing the Zero button.

 

3.  Data Collection

Click on the collect button and the snap your fingers sharply at the opening on one side of the tube.  The computer should display a graph which has several groups of sharp peaks.  Each group of peaks corresponds to an echo of the snap reaching the microphone, thus the distance between the peaks represents the time it takes the sound to make a round trip in the tube. If snapping your fingers doesn’t work, you can try clapping cupped hands together.  If LoggerPro doesn’t trigger data collection when you clap, contact your instructor.

 

1)  Use the examine button  to determine the round trip time of the sound in the tube.

 

T = ________________

 

2)  From your measured values for tube length and round trip time, determine the speed of sound in air.  Show your work in the provided space.

 

 

 

 

 

3)  The accepted value of the speed of sound depends on temperature but is usually about 340 m/s.  How does the value you determined compare to the accepted value.  Discuss the difficulties you had and any sources of error in determining the speed of sound using this method.