Anatomy of a Wave
Lesson 4 of 8
Objective: Students will be able to define waves in terms of amplitude, wavelength, frequency, and period.
During the previous lesson students found evidence of simple harmonic motion in pendulums. The goal for today's lesson is then to expand our knowledge of simple harmonic motion by defining the properties of waves (HS-PS4-1). Specifically, students go through a reading exploration activity that defines period, frequency, amplitude, and wavelength in both longitudinal and transverse waves (SP8). I start the class with a pop quiz before students move into a reading exploration activity. Today's lesson ends with students participating in a transverse wave demonstration as part of a misconception check.
Throughout this unit students have been defining and applying different concepts of simple harmonic motion in terms of pendulums and mass-spring systems, so today I start by giving students a pop quiz.
This short quiz is meant to formally assess if students can calculate the frequency of a pendulum, determine the restoring force of a mass-spring system, and describe the relationship between frequency and period. Since we are preparing for the AP Physics 1 exam, students get the same equation sheet that the College Board provides during the AP test. After students have had approximately ten minutes to complete this quiz, I collect their papers by lab table. Collecting the papers this way allows me to then pass the quizzes back out so that nobody receives their own quiz and we can informally grade the quizzes as a class.
When we grade quizzes as a class, I ask students to give each question an upward, downward, or leftward vector. An upward vector tells me that the student grading the quiz is 90% confident that the answer is correct, a downward vector tells me the grader is 90% sure the answer is incorrect, and a leftward vector tells me that the grader is unsure of the answer's correctness. I read the answers aloud from the answer key, allow students to make their vectors, and then recollect the quizzes. This informal grading opportunity allows students to get a general idea of how they did on the quiz, since they see the correct answers and are able to think back to how they answered on their own quiz.
These quizzes not only offer me information on how well my students are able to define and describe simple harmonic motion, but they also allow me to segue into today's activity. As part of the simple harmonic motion pop quiz, students should have used the relationship between period and frequency, as that relationship is included in the discussion of wave properties today. After class is over, I do go through and regrade these quizzes to better provide feedback for each individual student.
Because I know from our introductory activity that my students have knowledge of simple harmonic motion, students choose a partner and take a computer from the cart at the front of the room. As the computers are booting and after students have seated themselves next to their partner, I direct students to go to my website and download waves exploration. This document is a guideline that helps students identify the absolute minimum material they should record into their physics notebooks. Once they've opened the document, the students follow the link to the Waves & Wavelike Motion Section of the Physics Classroom. At this website and still in their pairs, students read through the text and use the document to guide them through the process of defining waves (in terms of wavelength, energy transport, period, etc.). I let students know that they can read aloud or individually, but the point of them being in pairs is to discuss and determine the most important parts of the text.
While students are working, I walk around with the answer key to ensure they are actively engaged in the learning process. To me, this means that they are on the proper website, reading or discussing some component of waves, and writing down a thorough set of notes. When I walk around, I spot check their written work and engage students in questions such as "Does the law of conservation of energy apply to waves?" or "Did you know there was a difference between longitudinal waves and transverse waves?" I also use this time to build rapport with students and attempt to make the material meaningful to the individual. For example, if I know a student is racing in a track meet soon, I might ask her "What is better to use as your cue to start a race: the drop the umpire's hand or the sound of the gun?"
As closure to the lesson today, I have a misconception on the board at the front of the room. I ask students to clear their desks and sit quietly for the activity because I really want them to think about what they know and how it applies to the statement on the board. When the students are ready, I read the statement and then ask students if they agree or disagree. I also remind them that in the world of AP Physics, an answer is not enough - we must justify everything. Then, I stay at the front of the room and just leave students to think for 2 or 3 minutes.
Today's misconception reads "Stadium waves are mechanical and longitudinal." and its purpose is to get students thinking about the definitions of transverse and longitudinal waves. Intentionally, there is no writing during this activity because I don't want to risk students copying down the misconception and then studying from it in the future.
After time is up, I ask student to raise their hands if they agree with the statement written on the board. I take a mental note of these students before asking one of them to explain their reasoning. Then, I ask a student who did not have his hand up to share why he disagreed with the statement. This process continues until a several students from both sides have shared their justifications. As an example, one student shared that "longitudinal waves move perpendicularly to the direction of a wave's motion," and a second student immediately responded by saying he thought "transverse waves have perpendicular particles."
Before the end of the class, I ensure that students understand that what is written on the board is a misconception. I ensure they understand this by making my students participate in doing a stadium wave. In forcing students to get up and active in their learning, they can see that the wave moved across the room yet as individuals they moved up and down. To bring this full circle and explain the misconception, I draw vectors on the board the show students how their motion is perpendicular to the motion of the wave. Then, I remind students that this is the definition of a transverse wave, so that while a stadium wave is mechanical, it is not longitudinal.