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Lab Assignment 11:  Properties of Waves

Instructor’s Overview

If you think carefully about it, we are immersed in waves.  The fact that you can turn on a radio and listen to music or news, or receive a cell phone call is evidence of the transmission and reception of waves.  These two examples involve transverse electromagnetic waves.  Having a conversation with a family member, friend, or colleague is made possible by longitudinal waves of sound.  Waves transmit energy and demonstrate interesting properties such as superposition (constructive/destructive interference) and resonance.  We’ll explore many of the properties of waves in this final lab of Physics I.

This activity is based on Labs 21 and 24 of the eScience Lab kit.

Our lab consists of three main components.  These components are described in detail in the eScience manual.  Here is a quick overview:

  • eScience Lab 21 Experiment 1: In the first part of the lab, you will use a Slinky® to create and visualize transverse and longitudinal waves.  You’ll experiment with wave reflection, superposition, and resonance.
  • eScience Lab 21 Experiment 2: In the second part of the lab, you will create your own wave source using a cork and a tub of water.  This experimental setup allows you to explore the Doppler effect.
  • eScience Lab 24 Experiment 2:In the final part of the lab you will explore the concepts of pitch and resonance by experimenting with water-filled bottles.

Take detailed notes as you perform the experiment and fill out the sections below.  This document serves as your lab report.  Please include detailed descriptions of your experimental methods and observations.

Experiment Tips

·  In general, read the lab questions below before running the experiments.  This allows you to keep an eye out for specific things as you run the experiments.

·  Make sure to run the Slinky® experiments on a hard floor.  Carpeted floors dampen the wave behavior and make the experiments more ambiguous.

·  For the Doppler effect experiment, it is easier to see the wave behavior with a larger tub of water.  Wave reflection from the sides of small tubs make the observations more challenging.

·  Narrow neck glass bottles work well for the sound experiments in eScience Lab 24.








Material and Methods


eScience Lab 21 Experiment 1: Slinky®

Based on your results from the Slinky® experiments, please answer the following questions:

1.  What happened when the transverse waves reached your partner’s end? Did the reflected wave stay on the same side as the one you sent? Draw a diagram showing the incoming and reflected waves.

2.  Did the waves go any faster or slower when you tried a variety of amplitudes? Explain how this agrees or disagrees with the equation for a transverse wave’s velocity.

3.  What did you notice about the speed of the longitudinal waves compared to the transverse waves?

4.  Explain what happened when you and your partner both sent waves on the same side. What kind of interference took place?

5.  What happened when waves on opposite sides passed each other?

6.  How did shortening the length of the spring affect the resonant frequencies? How does this confirm the relationship v = λ f when velocity is constant? (Hint: a shorter spring length means smaller wavelengths for each standing wave).

7.  Using this knowledge, explain how musical instruments create higher and lower tones. Use a string instrument as an example.

eScience Lab 21 Experiment 2: Doppler Effect

Based on your results from the cork and water experiment, please answer the following questions:

1.  Draw a picture of a moving source and the waves surrounding it according to what you observed in this experiment. How does the spacing of the wavefronts in front of the source compare to those behind it?

2.  Imagine a small observer is positioned in front of cork in your picture above. As the cork approaches, the observer measures the wavelength of the waves passing by. How does this wavelength compare to that measured from behind the source?

3.  Imagine that this same observer measures the frequency of the waves instead of wavelength. How does the frequency measured in front of the source appear to the observer compared to the frequency measured from behind?

4.  How do these results help explain why a car’s engine sounds different as the car approaches you compared with after it passes?

5.  The Doppler effect is present in light waves as well. As you will learn in Physics II, red light has a lower frequency than blue light. Based on your observations in this experiment, what can you speculate about the motion of a distant star that appears “red‐shifted” to astrophysicists? (The light appears more red than expected.)

eScience Lab 24 Experiment 2: Pitch and Resonance

Based on your results from water bottle experiments, please answer the following questions:

1.  Did the pitch of the noise made by striking the bottle get higher or lower as you filled the bottle with water?

2.  Did the pitch made by blowing across the top of the bottle go up or down as you filled the bottle?

3.  Why is there a difference between these two noises? In your answer, comment on the source of the noise in each case (i.e. What is vibrating?).

4.  Using what you know about harmonics and resonance, explain how different pitches are created when you blow across the bottle opening with different liquid levels.

5.  What wave property allowed you to hear noise through your bottle in step 2 of Procedure 2, and how does the sound transmit from one bottle to the other? Did the pitch sound the same as the one made by your partner?



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