Lesson One : Activities and Quizzes
Activity 4: Transverse and longitudinal waves measuredRequired materials: A Slinky® (preferably a large metal one), a long ruler, a stopwatch and three volunteers.
Procedure: First, have two of the students pull the Slinky® taut between them. Have one of the students shake his end up and down at a constant rate to create a series of waves on the Slinky® (these waves will be stationary; they are called standing waves). The rate at which the Slinky® is vibrated should not be too fast. Point out to the students that these are transverse waves.
Have the third student take the ruler and measure the distance between the peaks of the wave (be sure the rate at which the wave is being vibrated doesn’t change). This value is the wavelength of the wave.
Next, have the third student take the stopwatch and count the number of times the student who is vibrating the Slinky® is moving his hand up and down (i.e. up and down, 1; up and down, 2; up and down, 3; etc.) in one minute. This number (given in cycles per minute) represents the frequency of the wave. Now have the student shaking the Slinky® speed up and notice how the wavelength of the standing wave decreases. If desired, the student may measure the new wavelength and frequency to confirm the change.
Next, have the students lay the Slinky® on the floor and pull it taught between them. Now have the other student move her end of the Slinky® back and forth to generate pulses that move along the Slinky®. Point out to the students that these are longitudinal waves. While keeping the rate of vibration constant, have the student with the ruler measure the distance between the center of one pulse and the center of the next. This is the wavelength of the wave. Then have the student use the stopwatch and count the number of times the student generating the waves moves her hand back and forth in a minute (i.e. back and forth, 1; back and forth, 2; back and forth, 3; etc.).
Again notice that if the frequency of the vibration increases, the wavelength (the distance between the pulses will decrease) and vice-versa.