Newton's Third Law

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Students will demonstrate an understanding of Newton’s third law by crafting explanations in the context of a college level text.

Big Idea

Constructing explanations of Newton’s third law is a great way to learn and practice physics.


This lesson addresses the HSA-REI.A.1 and HSF-BF.A.1a standards as a way to effectively compose a logical understanding of Newton's third law in the context of a college level physics text. Students research concepts related to Newton's third law using the NGSS Practices of Asking Question (SP1), Developing and Using Models (SP2), Using Mathematical and Computational Thinking (SP5), and Obtaining, Interpreting and Communicating Information (SP8). Students begin with ann EdPuzzle which is a set of video notes with embedded pause points as a whole class. Student pairs then use the class digital text and a credible to ask questions and answer those questions in the context of the textbook. During the closure activity at the end of this lesson, I ask students to construct a definition to demonstrate an understanding of the concept of "Newton's third law". 

I assess student understanding throughout the lesson using informal check-ins and assess each student's work at the end of the school day. I want students to learn to integrate information from various points of this course into a coherent summary on the nature of Force Interactions. This relates to (SP6) because students have to leverage skills like note taking to construct an explanation of force pair interactions within a system.  One goal of this lesson is to help students learn that synthesizing information from more than one credible source is an effective way to communicate scientific information about the concept of "Newton's third law".


10 minutes

This portion of the lesson begins with a routine where students write the objective and additional piece of information in their notebooks as soon as they enter the classroom. I project a slide with the date, the objective and an additional prompt  on the interactive whiteboard with a red label that says "COPY THIS" in the top left-hand corner. Sometimes the additional prompt is a BIG IDEA for the lesson or the Quote of the Day or a Quick Fact from current events that is related to the lesson. The red label helps my students easily interact with the information as soon as they enter the room and avoids losing transition time as students enter the classroom.

Today's additional piece of information is a Big Idea which states that constructing explanations of Newton's third law in context is an essential skill for learning and practicing physics. The objective of the bell-ringer is to give students a clear understanding of the focus of today's lesson.  I choose to use active reading instead of a lecture because I want students to learn that constructing explanation using technical text is useful for studying and learning about Newton's third law.

EDpuzzle: Newton's Third Law

15 minutes

After students complete the bell-ringer activity, students construct explanations of Newton's third law of motion using information from a set of video notes. I include a set of notes that I project at the interactive whiteboard in the front of the room for this section below. This part of the lesson focuses on the conceptual model for Newton's third law. For the first ten minutes, I play the notes at the front of the room for the entire class and pause at the pause points I embed as green question marks in the video below.

During the first ten minutes, students take notes in their notebooks. I ask students if they have any questions or concerns about the methods discussed in the video. We have a whole class discussion for 2-4 minutes. Some student questions include, "Don't we only look at the forces acting on a single object?" and "If the forces happen at the same time, why do we call them action and reaction forces?" During the last minute of this section of the lesson, I email this video and notes to the entire class so that students can watch, pause and replay the video outside of class. During the next section, students are given a set of readings and guiding questions related to these notes to create more detailed notes in pairs.

Active Reading: Newton's Third Law

40 minutes

In this section of the lesson, I ask students to construct an explanation of Newton's third law using this handout and our digital textbook. I introduce different notetaking strategies in an earlier lesson that I now ask students to use while take notes from our openStax digital textbook and the physics classroom website on the concept of Newton's third Law. Students spend the next 20 minutes taking notes by:

  • Writing headings as questions
  • Describing concepts, graphs, charts and tables that are related to understanding the concept of "Newton's third law"
  • Showing the connection between basic physic concepts like acceleration and momentum conservation to Newton's third law.

As students are taking notes, I walk around checking in with them. The purpose of this assignment is to have students use information from multiple sources and perspectives, much like scientists construct explanations of complex topics. Students spend 10 minutes discussing their notes with their elbow partners and then making additional notes. Click here for one example of student work. At the end of the week, students use these notes during an open notes quiz. This task helps students illustrate the depth of their current understanding of Newton's third law of motion. 


10 minutes

This closure activity this section asks students to spend 5-10 minutes working individually to complete a free write to answer the question "Which object exerts a larger force during a collision an MTA bus or a messenger bike?" in their notebooks. During a free write activity students write with a utensil of their choice on a topic or answering a guiding question without erasing or editing their responses for grammar or correctness. Student responses include: "Neither object exerts a larger force, this would break Newton's third law.", "The MTA bus exerts a larger applied force, it has more mass, but I think the net force each object on the other is equal", "This is a pair of action-reaction forces so the forces are equal and opposite and occur at the same time". I like this activity because students produce a working definition of an important but often misunderstood concept of equal and opposite Force pairs. This helps drive home the idea that physics models are sometimes opposite to conventional logic. 

To wrap up this section of the lesson, I ask students to look at the Minds on Physics modules (Newton's Laws:Assignment 12) that I post on the class Edmodo wall for homework. I also ask students to share their medal success codes with me by midnight on Thursday morning to meet the HW deadline. Each module has a progress bar and a success bar. As students answer questions correctly their progress bar goes up. If students begin to answer questions incorrectly their health bar decreases. If a student's health bar reaches zero before completing the module they have to attempt the module again. The content remains constant, but the questions change either in order or phrasing each time a student attempts a module. Once student progress reaches 100% on a module students receive a medal success code.

If a student demonstrates mastery with little to no evidence of stumbling he or she receives a gold medal success code. If a student reaches mastery but stumbles in one or two instances, her or she receives a silver medal success code. Students who receive a silver medal success code are given the opportunity to continue practicing to attain a gold medal success code. Students record these alphanumeric codes on a record sheet which correspond to either gold or silver medals on a Minds on Physics module.