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Learning Introductory Physics with Activities

Paul J. Emigh, Rebecka Tumblin, Kathryn Hadley, Danielle Skinner

Section 7.6 Boundaries and Discontinuities

Exercises Warm-up Activity

Play with this simulation for a few minutes.

1.

Write an interesting observation you made using the simulation. Try to connect your observation to something you have experienced in your everyday life.

2.

If you have not done so already, follow these steps in the simulation:
  • Click on the Pulse button and reduce the damping to zero.
  • With the right end fixed, click on the green Pulse button once and observe what happens.
  • Develop a rule for what happens to a wave pulse when it reflects from a fixed end.
  • Click on the yellow Restart button.
  • With the right end loose, click on the green Pulse button once and observe what happens.
  • Develop a rule for what happens to a wave pulse when it reflects from a loose end.
Answer.
Reflections from fixed ends tend to flip over, while reflections from free ends do not.
Figure 7.6.1. A wave reflecting from a fixed end.
Figure 7.6.2. A wave reflecting from a free end.
The image below shows a wave pulse moving from left to right in two different situations: from a region with high density to a region with low density (top) and from a region with low density to a region with high density (bottom).
Figure 7.6.3. A wave pulse approaches a boundary between two strings in two different configurations.

3.

Predict what you think will happen after the wave pulse reaches the boundary in the top experiment.
Answer.
Figure 7.6.4. A wave encountering a discontinuity from a high density to a low density.

4.

Predict what you think will happen after the wave pulse reaches the boundary in the bottom experiment.
Answer.
Figure 7.6.5. A wave encountering a discontinuity from a low density to a high density.

References References

[1]
Animations courtesy of Dr. Dan Russell, Grad. Prog. Acoustics, Penn State. licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License. For more information see https://www.acs.psu.edu/drussell/demos.html
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