Workout: Superposition of waves in 1D


Superposition of waves in 1D


the PhET simulation, Wave on a string

Set up

Select "Pulse" and "Fixed end", turn on "Rulers" and "Slow motion". Set the Tension to "Medium" and the damping to "None". Your screen should look like the screen below.

Answer these questions

Push the button to send a pulse down the string and note what happens. We'll not study why this does this here, but we'll use it as an opportunity to explore what happens when two pulses heading in opposite directions meet.

1. Start a pulse moving toward the clamp. When the back end of the pulse reaches 4 cm, start a second pulse. When the first pulse has fully reflected from the clamp press pause. Your screen should have two non-overlapping pulses: a positive one on the left side of the string that will move to the right, and a negative one on the right side of the string that will move to the left. (If you have trouble getting this to happen, press "Restart" and try again.) What do you expect to happen? Will the two pulses annihilate each other and disappear? Will they bounce back from each other? Will they pass through each other, each pulse totally ignoring the other? Explain why you think so and restart it to see if it agrees with your expectation. 

2. Set up the same situation as in 1 with the two pulse stopped and approaching each other. Now step the pulses towards each other using the small forward arrow with the bar. (This moves the sim one step. You can hold it down to take multiple steps.) Stop when they are completely overlapping and the string is flat. The beads are all in the same position as when there is no pulse. How do they know to recreate the correct two pulses? 

Joe Redish, Spring 2017

Article 698
Last Modified: June 3, 2019