Waves on a String
Lesson 5 of 8
Objective: Students will be able to predict the behavior of waves through varying medium and at reflective endpoints.
Now that students have a clear understanding of the anatomy of a wave from the previous lesson, today's goal is to apply those concepts and explore how waves move through a string (HS-PS4-1). Students begin by watching a short video on the speed of transverse waves moving through a string. Then, they use a simulation to visualize how a transverse wave changes with free and fixed ends (SP2, SP4 & SP7). As the students work in pairs, they see which variables affect the speed of a wave and explore standing waves. Finally, the lesson ends with students applying their newly learned knowledge in a practice makes perfect closure activity.
Because there are so many resources out there and my students are active for the rest of the lesson, I decide to use this short video to introduce the speed of transverse waves in a string. While we discussed forces and tensions in a previous lesson, this lesson requires students to apply their knowledge specifically to strings that propagate waves. It is the goal of this video strategy to provide students with a concise and straight-forward look at the speed of a transverse wave in a rope. The video has detailed diagrams and offers an example similar to what students experience in the next part of this lesson. Also, I remind students that multiple-part problems, similar to the one shown in the video, are likely to be found on free-response section of the AP Physics 1 exam.
Before I start the video I make sure my expectations are clear: students need to be sitting quietly, listening and watching the video, and taking notes on meaningful material. To me, meaningful material includes any reference to previously learned concepts, equations, vocabulary, and examples. While I do not collect their notes, these are an essential resource as students work through the simulation later in the lesson. I am telling my students these expectations as I'm on my way to start the video.
Waves on a String Simulation
After we've finished watching the video, students use a computer simulation that allows them to be more active in their learning. The simulation allows students to apply scientific vocabulary when discussing waves moving through different mediums or meeting different types of endpoints. Specifically, students start by exploring the Wave on a String Simulation and reviewing the anatomy of a wave. Then, students use the simulation to change the speed of a wave and investigate how that speed changes due to different factors such as tension and damping. The simulation also gives students an opportunity to differentiate between free and fixed endpoints. Finally, students are able to identify standing wave properties by using specific settings on the simulation.
Before we start the activity, I assign partners using the random student generator that already has my students' names loaded. By displaying the random generator on the front board, there is an element of suspense as the partners are assigned. Partners work best for this activity so that each student can be actively engaged throughout the work time. Because this activity does not need to be completed outside of class, I feel comfortable in choosing the pairings for my students.
We use MacBook Pro's in my district, so my students are familiar with how the computer and cart organization works. After each pair has been assigned, the students move to sit near each other, push their desks together, and grab a computer. As the computers are booting I pass out the waves on a string activity so that each student receives a copy. This document directs them to the proper simulation and guides students in their learning so that they are confident in what material needs to be understood. However, the answers to the questions, data tables, and graphs need to be completed on a separate sheet of paper.
While students are working, I walk around to ensure they are actively engaged in the learning process. This means that they are on the proper website, reading or discussing some component of the simulation, and writing down appropriate information from the simulation. When I walk around, I'm spot checking their written work and engaging students in questions such as "Have you had time to explore the effect of damping yet?" or "What do you think would happen if you just slightly modified one of those settings for a standing wave?"
The students' written answers are the most important part of this activity, so I ensure they are thinking through how they will prove any claims they make. I do a lot of walking and questioning throughout their work time to ensure they can justify anything that has been written down. The AP Physics 1 exam places a lot of emphasis on justifying thought processes, so my goal in our dialogue is to practice with students how to successfully justify their arguments.
When there is approximately 10 minutes prior to the end of class (5 minutes left of the time I've allowed for this activity), I ask students to put the computers back on the cart and return to their seats. I also tell them at this point that the lab is due at the start of the next class meeting. Once everyone is back in their seats, we are ready to move into the closure activity for our lesson.
As closure to today's lesson, students engage in a practice makes perfect - wave activity. I've chosen this specific problem to share with students because it is quite similar to the problem students saw in this lesson's video introduction. Also, the problem gives students an opportunity to apply their newly learned knowledge about waves on a string quantitatively - something they will need to do both on the unit test and AP Physics 1 exam.
The closure officially starts when I read the problem aloud after it has been projected on a screen at the front of the room. After I finish reading, I highlight the diagram and point out measurements, such as the string length. I then ask students if they have any questions about the narrative or diagram before giving students a minute or two to work through the problem individually. During this individual work time, I expect students to write down given information and start to solve the problem in their science notebooks. While I won't collect their work, students understand that this problem, their work, and the solution are all study resources.
Once students have finished to work alone, they then have the opportunity to work with someone seated near them for approximately two minutes. During this time, students can discuss possible methods of solving the problem, possible errors in calculations, or final answers to the practice problem.
Finally, I share my solution to the problem with the students. When I'm sharing the solution, I display my work on the front screen, adjacent to the question. As the solution is displayed, I verbally walk my students through my thought process, use of equations, substitutions, and the final answer. As I do this, I allow students to interrupt me with questions and make sure to check in several times by asking "How do you feel about this?" or "Does this make sense?" At this point in the year, my students aren't shy and speak up when they need clarification. Once students feel confident in the solution and all questions are answered, class ends and we are are ready to move into wave reflection & interference at the start of the next class.