Module 8—Mechanical Waves

Transverse and Longitudinal Waves

 

A coil spring is shown in this image.

© Jonathan Brizendine/shutterstock

There are two general classifications for waves—transverse and longitudinal. A toy spring, seen here, is perfect for demonstrating both types of wave motion. If you wiggle one end of the spring up and down, you will produce a transverse wave. Stretching it out and quickly compressing one end of it will generate a moving longitudinal wave. You will use a simulation to compare and contrast both types of waves.

 

transverse wave: a wave in which the medium moves at right angles to the direction of the wave

 

longitudinal wave: a wave in which the medium moves parallel to the direction of the wave

 

 

 

 

 


 

Open the Travelling Waves simulation. Select the “Transverse” option () and, click “Play.” Carefully compare the motion of the red dot and the motion of the wave itself. You will notice that these motions are not the same. The small red dot represents the medium through which the wave is passing. Spend a few minutes experimenting with different settings for both the wavelength and the frequency, and observe the behaviour of both the wave and the medium before attempting the following tasks.

Self-Check

 

SC 4.

  1. Describe the motion of the red dot (the medium). Does it move horizontally or vertically?

  2. Describe the motion of the wave itself. Does it move horizontally or vertically?

  3. In what way is the motion of the red dot (medium) distinctly different than the motion of the wave?
Check your work.
Self-Check Answers

 

SC 4.
  1. The red dot moves vertically up and down.
  2. The wave moves horizontally to the right.
  3. The medium moves perpendicular to the wave.

 

On the simulation, select the “Longitudinal” option (), and click “Play.” Once again, carefully compare the motion of the red dot and the motion of the wave itself.

 

Self-Check

 

SC 5.

  1. Describe the motion of the red dot (the medium). Does it move horizontally or vertically?

  2. Describe the motion of the wave itself. Does it move horizontally or vertically?

  3. How does the distance that the wave travels compare to the distance that the red dot moves over a few seconds of time?

  4. In what way is the motion of the red dot (the medium) distinctly different than the motion of the wave?
Check your work.
Self-Check Answers

 

SC 5.

  1. The red dot moves horizontally.

  2. The wave moves horizontally, travelling in only one direction.

  3. The wave travels the same amount of distance as the red dot in any time interval because they both move at the same speed. However, the wave travels in only one direction whereas the medium (red dot) travels back and forth.

  4. The red dot moves back and forth, but the wave travels in only one direction.

 

You have just described a key point about wave motion. The motion of the wave and the motion of the medium through which the wave passes are two distinct and different motions. These motions are summarized below.

 

Transverse Wave Motion

 

 

The medium vibrates perpendicular (transverse) to the direction of the wave motion.

 

Longitudinal Wave Motion

 

 

The medium vibrates parallel to the direction of the wave motion.

 

Comparing the Motion of the Medium with the Motion of the Wave

 

Does the medium travel as fast as the wave that passes through it? The simulation will now be used to compare the speed of the medium with the speed of the wave.

 

On the simulation, select the “Transverse” option (), and set the frequency to 0.5 Hz and the wavelength to 90.0 m. Carefully position the red dot at its lowest point. Do this by clicking “Play,” and then immediately click “Pause.” Use the “Forward” or “Back” buttons to move the dot to the desired position. Record the time shown at the top of the screen in the table below. Use the “Forward” button to move the dot to its highest point. Record the time again.

 

Self-Check

 

SC 6.

  1. Complete the following table.

    total distance dot travels (1 grid square = 10 m)

     

    start time (s)

     

    end time (s)

     

    time elapsed (s)

     

    average speed of dot (in m/s)

     


  2. How fast is the medium moving, and in what direction is it moving?
Check your work.
Self-Check Answers

 

SC 6.


  1. total distance dot travels (1 grid square = 10 m)

    140 m

    start time (s)

    5.0 s

    end time (s)

    13.0 s

    time elapsed (s)

    8.0 s

    average speed of dot (in m/s)

    18 m/s


  2. The medium is moving upwards at 18 m/s.

 

Next, carefully follow a wave crest. Do this by clicking “Play” and then immediately clicking “Pause.” Use the “Back” button to take the time back to zero. Record the position of the first crest by counting the squares. Record the value in the table below. Click “Play” and then a few seconds later, while the crest is still visible, click “Pause.”

 

Self-Check

 

SC 7.

  1. Complete the following table.

    initial position of first crest (1 grid square = 10 m)

     

    final position of first crest (m)

     

    total distance wave travels (m)

     

    initial time (s)

    0.0 s

    time at end (s)

     

    time elapsed (s)

     

    average speed of wave (in m/s)

     


  2. How fast is the wave moving, and in what direction is it moving?
Check your work.
Self-Check Answers

 

SC 7.


  1. initial position of first crest (1 grid square = 10 m)

    68 m

    final position of first crest (m)

    258 m (varies )

    total distance wave travels (m)

    190 m (varies )

    initial time (s)

    0.0 s

    time at end (s)

    33.0 s (varies)

    time elapsed (s)

    33.0 s (varies)

    average speed of wave (in m/s)

    5.76 m/s


  2. The wave is moving to the right at 5.76 m/s.

 

Your observations from this simulation show that the medium and the wave move in distinctly different ways, at different rates, and sometimes in different directions.

 
Watch and Listen

 

Watch this transverse vs. longitudinal video clip to compare the properties of longitudinal and transverse waves.




On the surface, water waves appear to be transverse waves. A closer look at the medium, however, shows that water waves are, in fact, neither transverse nor longitudinal. Watch this water waves video clip to observe the motion of a water medium when a wave passes by.




Read

 

Read “Transverse and Longitudinal Waves” on pages 401 to 407 of your textbook to find out even more about the topic.

 

Module 8: Lesson 1 Assignment

 

Remember to submit the answer to TR 2 to your teacher as part of your Module 8: Lesson 1 Assignment.

 
Try This

 

TR 2. Recall the earlier definitions given in this lesson for the terms listed in the table below. How could each of these terms be defined differently based on the type of wave? A quick diagram may help you explain these.

 

Term

Transverse Wave

Longitudinal Wave

crest

 

 

trough

 

 

amplitude

 

 

wavelength

 

 

 

Self-Check

 

SC 8. Complete question 2 of “Practice Problems” on page 407 of your textbook.

 

Check your work.
Self-Check Answers

 

SC 8.

 

Given

 

v = 3.60 m/s              
l = 2.50 m

 

Required

 

the time required to produce the pulse (t)

 

Analysis and Solution

 

Use the formula to find the time. The length (l) is equal to Δd.

 

     

 

Paraphrase

 

The time required to produce the pulse is 0.694 s.