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Learning Introductory Physics with Activities

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

Section 17.2 Boundaries and Discontinuities

Exercises Activities

Play with this simulation for a few minutes.
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1.

Write an interesting observation you made using the simulation. Try to connect your observation to something you have experienced in your everyday life.
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2.

If you have not done so already, follow these steps in the simulation:
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  • Click on the Pulse button and reduce the damping to zero.
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  • With the right end fixed, click on the green Pulse button once and observe what happens.
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  • Develop a rule for what happens to a wave pulse when it reflects from a fixed end.
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  • Click on the yellow Restart button.
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  • With the right end loose, click on the green Pulse button once and observe what happens.
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  • Develop a rule for what happens to a wave pulse when it reflects from a loose end.
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Answer.
Reflections from fixed ends tend to flip over, while reflections from free ends do not.
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Figure 17.2.1. A wave reflecting from a fixed end.
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Figure 17.2.2. A wave reflecting from a free end.
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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).
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Figure 17.2.3. A wave pulse approaches a boundary between two strings in two different configurations.
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3.

Predict what you think will happen after the wave pulse reaches the boundary in the top experiment.
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Answer.
Figure 17.2.4. A wave encountering a discontinuity from a high density to a low density.
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4.

Predict what you think will happen after the wave pulse reaches the boundary in the bottom experiment.
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Answer.
Figure 17.2.5. A wave encountering a discontinuity from a low density to a high density.
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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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