Applying A Problem-Solving Protocol to Newton's Third Law Problems

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Students will demonstrate their understanding of Newton's third law by creating visuals that apply the G.I.R.L.S. protocol to a set of problems.

Big Idea

The G.I.R.L.S. protocol is a great way to study and learn about Newton's third law.


The goal of this lesson is to help students make connections between Newton's third law and the behavior of objects during an interaction. This lesson addresses the HS-PS2-1 and HSA-REI.A.1 standards because it asks students to use their understanding of the forces to solve a series of problems in a step by step manner. It aligns with the NGSS Practices of Developing and Using Models (SP2), Using Mathematical Reasoning (SP5) and Obtaining, Evaluating and Communicating Information (SP8) for Science because students use their prior knowledge of forces to answer the embedded questions in an EdPuzzle on the Application of Newton's third law. Students then take notes while following along with an example problem I lead at the interactive whiteboard at the front of the room. Then students use the information from the EDpuzzle and example problem to solve a set of practice problems and create a summary on the application of Newton's third law. During the closure activity at the end of this lesson, I ask students to discuss the most important and challenging parts of today's lesson on forces and 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 conceptual understanding of Newton's third law. This relates to (SP6) because students have to leverage skills like note taking and using logic to recognize patterns while developing an understanding of how objects accelerate. One goal of this lesson is to help students learn how to change the mathematical model of Newton's third law to address a series of real world scenarios. 


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 the G.I.R.L.S. protocol is a great way to learn and study Newton's third law. The objective of the bell-ringer is to give students a clear understanding of the focus of today's lesson.  I want students to learn that applying Newton's third law can be helpful in solving real world problems.

EDpuzzle: Newton's Third Law

15 minutes

Within this lesson, I review a mathematical model for Newton's third law of motion. I include a set of notes that I project at the interactive whiteboard in the front of the room. This part of the lesson focuses on applying the mathematical model for Newton's third law to solve for the acceleration and tension forces acting on a system of masses connected by ropes. 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 each video.

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 queries include, "Does this work if two people are pushing on the same object?", and "What happens one person pulls downward at an angle?" During the last minute of this section of the lesson, I email these video notes to the entire class so that students can watch, pause and replay the video at their convenience. During the next section, students work in pairs to solve a set of practice problems that connect to Newton's third law of motion.

Newton's Third Law Practice Problems

40 minutes

During the first ten minutes of this section, I project an Example Problem on the interactive whiteboard at the front of the room. Students spend about eight minutes taking notes and then we have a brief discussion about the example problem. I also provide students with Chromebooks and post the problems on our class Edmodo wall so students can work at their own pace. Some student questions include, "How do we know which objects are in each system?" and "Why isn't the force from the floor a negative one?" After the discussion is over I circulate and students spend the next twenty minutes solving the provided Independent Practice Problems. Click here to see an example student solution to an independent practice problem.

After twenty minutes pass, I ask students to work in teams of 2 -4 creating a summary on the application of Newton's third law of motion to real world scenarios. Each summary should include:

  • A problem 
  • The physics of the problem
  • A step-by-step solution 
  • Tips for peers on how to analyze similar problems

Students may create summaries in the lab notebooks or in a shared google document. Students summarize one of the questions from today's lesson or create a similar problem in their groups to explain in their summary.  I remind students of the digital resources (Chromebooks, openStaxEdpuzzles) available to them and then ask for resource managers to obtain Chromebooks from the resource station at the front of the room. 

Once we discuss the requirements for the summaries, students begin working in groups of 2-4 at a location of their choice to summarize the connections between the net force exerted on a system and the acceleration of an object. I circulate the classroom with my classwork assessment clipboard and make notes of groupings and answer clarifying questions for students.  At the end of this section, I pause and ask students to return the materials they used during this section to the front resource station. A resource manager returns the Chromebooks to the laptop bins so that they are readily available the next time the materials are needed. 


10 minutes

This closing activity asks students to write a headline that encapsulates the most important part of today's lesson in their notebooks. Some student responses include, "It's easy to remember that there is a reaction force and to draw it on the free body diagram" and "Newton's third law makes it easy to find a reaction force in a system". I like this activity because it gives students a chance to voice their current level of understanding and their thinking about their learning processes based on today's lesson. 

I check student responses to this closure to determine whether students are proficient in the understanding of the connection between the mathematical model for Newton's third law, free body diagrams and the acceleration of an object at the beginning of the next lesson. To wrap up this section of the lesson, I ask students to look at this tutorial and practice questions that I post on the class Edmodo wall for homework.