Newton's Third Law

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Students will be able to define and identify equal and opposite forces in the context of Newton's Third Law.

Big Idea

Students define Newton's Third Law and then find evidence of it in a 2013 science fiction thriller movie, Gravity.


Since this lesson falls on a half day, it is a bit shorter than other lessons. My students have already learned the fundamental concepts of Newton's First and Second Laws, but this lesson is flexible in that in can be placed in other parts of the Newton's Laws Unit. Other than asking students to recall the vector nature of forces, this lesson requires very little prior knowledge.  

The goal today is to define Newton's Third Law, but the lesson also builds upon what students already know about Newton's Firs and Second Laws (HS-PS2-1). I start the lesson by assessing students' prior knowledge with a one-minute essay. Students will then take some brief notes on the third law (SP5) before identifying examples of it in a movie trailer (SP6).

One-Minute Essay

10 minutes

To start class today, students come into the room and take out a blank sheet of notebook paper. On the board I've written "What is Newton's Third Law?"

This is a 1-minute essay, where students get only 1 minute to individually respond to the question. The goal is to get students thinking about what they already know from previous science classes, since they need to have action/reaction pairs at the forefront of their minds for today's lesson. I use a stopwatch, but because some of my students need a little time to process I usually give them about 90 seconds to complete their responses.

After the time is complete, I randomly select a student to share his or her essay and then we continue around the room until everyone has shared. I try remain a facilitator during this process and let students consider the responses of their peers. For example, if a student makes an assumption that is not valid, I let another student speak up and question the invalid statement. I also encourage students to write down any valid ideas that they had forgotten to include. This is also an opportunity for me to informally observe my students and get a sense of how well they know Newton's Third Law.

Notes on Newton's Third Law

15 minutes

It's time for students to take out their notebooks to copy down the definition of Newton's Third Law before working through some sample problems. My students are operating under the expectation that they must write down key points from the presentation. This expectation of how to take notes has been outlined and ingrained in their learning since freshman year. Because these are AP students, how they organize their notes (notebook, binder, etc.) is a decision the individual student gets to make. I assume at this point in their high school careers they have an established system to stay organized.  

I display the Third Law Presentation to help the students understand what they need to write down. Not only do my notes serve as a guide for students, but they also include examples. These examples provide students with something to refer back to when doing homework problems or studying for the unit exam. I have a hard copy of the slides in my hands as I circulate throughout the room, which includes notes and answers to the sample problems (viewable when the PowerPoint is downloaded). These notes help me to stay focused and ensure I mention the highlights as we progress through each slide.

I usually have a lot of interaction with my students throughout the presentation, and it's purposely short in content because my goal is to emphasize the application of action-reaction pairs. The notes are meant to give students a sense of how action-reaction forces can be applied with what students already know from Newton's First and Second Laws. After I define the third law for students, I show them the first example, give them about three minutes to process the question, and then go through the solution slowly and deliberately. For the second example, I give students about five minutes to process before sharing the solution. As I go through each solution, the students will ask questions and participate in my work to stay engaged the entire time.     

Newton's Third Law in "Gravity"

10 minutes

The goal of this lesson closure is to get students to identify Newton's Third Law in a movie that is current and relative to the students. Many of my students have seen "Gravity," so it is a great way to make physics meaningful and interesting. I start by asking students to clear their desks because I want them to enjoy being captivated by this awesome video clip. I let the video play through and then give students a few moments to debrief about what they saw.

The few moments of debriefing may turn into a full class discussion. My students are eager to discuss the errors in physics in this clip (and, for those who have seen the movie, at other points in the movie.) As a teacher, it is super neat to see my students so excited in applying some of what we've already discussed in this course.

After our discussion, I ask students to take out a scratch piece of paper and to watch the video clip a second time using a different lens. I want students to look for examples of Newton's Third Law when they watch the clip again, and I ask them to write these examples on the scratch piece of paper.

When the video has finished playing through the second time, I choose volunteers to share their examples. There are such a large number of opportunities for students to identify Newton's Third Law, so the opportunity to have a rich discussion and bring the lesson full circle offers a nice closure. As students share their examples of Newton's Third Law, they listen to each other, make comments about other students contributions, and hopefully learn how many action-reaction pairs are possible in just a short video clip. The discussion continues to flow naturally until the bell rings, and which point students are dismissed and ready to move on to their next class.