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# Newton's 3rd Law, Day 1

Lesson 7 of 9

## Objective: Students will be able to identify action and reaction forces for a pair of objects by labeling them on a diagram and will be able to identify which object will have greater acceleration.

*60 minutes*

The goal of this lesson is to introduce students to Newton's Third Law and action and reaction forces. Students learn how action and reaction forces can be depicted on a free body diagram and how to identify the forces in real-world situations. This is one of my favorite lessons of this unit because students find their own videos that demonstrate Newton's Third Law.

To start out class, I ask students to take out their Unit 3 Learning Targets to complete a second look at the targets. Students read through all of the targets and rate on a scale of 1-4 how they feel about their knowledge and ability to complete that learning target. We use the Assess Yourself labels of (1) Novice, (2) Apprentice, (3) Practitioner, and (4) Expert. They silently read through the targets and rate themselves on the targets using a symbol the class chooses (ex. circle, square, star, smiley face). When all students have read through all of the targets and rated themselves on the learning targets, I tell students to focus on the learning targets that are still not in the 3 or 4 rating. By looking over these learning targets in the next day or two, students should be able to practice review questions on the review sheet and ask questions of me in class before the upcoming test.

#### Resources

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After students finish looking over all of the learning targets, I direct them to look at the learning targets that we cover in this lesson (#7-10). I do this to help students to see a connection between the learning targets and the activities in class.

I teach Newton's Third Law with Guided Notes because I like to provide a reference tool for the students to look at. I front-load the guided notes section with a mini-lecture on Newton's 3rd Law.

After the mini-lecture, I show students one of the examples on the next page of their notes. Students see a baseball hitting a bat. The action and reaction forces are already written out in this example, so I take this opportunity to teach students how to label the forces. I make sure to point out that the objects in each force label are the same two objects. Then I ask students "Which has a greater force?" On this first example I get students that say the bat because that is the object the is being moved, but I remind them that action and reaction forces are always equal and opposite. The last question is "Which object has a greater acceleration?" Students remember back to the example on the notes page and say the ball because it has less mass.

After the first example, students complete the next two examples with the students at their tables. When 5 minutes have passed, I direct the students' attention back to the projector screen and as students to volunteer their answers for the problems just completed. I try to get a different student for each box so I get multiple students participating. After we have gone over the two examples, I do a Thumbs Up, Thumbs Down assessment to see if the students would feel comfortable completing these questions on their own.

Then I show students a recoil video and ask them to complete all of their boxes for this row individually.

The final example I show students is with two circle spring scales, as shown below, linked together. I have a small student and a large student pull and show the class how the numbers read the same. I try to pick a big football player and a small petite girl for this demonstration so they can see no matter the difference in mass, the force is always the same.

The notes and the examples should look like the N3 Law Notes Key when finished.

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#### Student Video Activity

*20 min*

After the notes about Newton's Third Law are completed, I have students find 3 YouTube videos that show Newton's Third Law. I ask them what types of videos might show it best and students reply sports, car crashes, cannons, etc. Before giving them time to work, I put the three questions on the board that students must answer for this activity:

(1) What are the action and reaction forces?

(2) Which force is greater?

(3) Which object has a greater acceleration?

For this activity I ask them to record their answers and pictures of the situation on a sheet of paper and to bookmark the video links on their Chromebooks. Students work in pairs with the person sitting next to them to complete this activity. I do this activity because it helps students practice identifying action reaction forces as well as the other concepts with Newton's Third Law; it also allows students to use higher level thinking when they are looking through the videos. One group's Student Videos are shown below. They keep those video question sheets until the next class period where students will share-out some of their examples to the class.

#### Resources

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- LESSON 1: Intro to Newton's First Law of Motion
- LESSON 2: Inertia Day
- LESSON 3: Inertia Demo Day
- LESSON 4: Free Body Diagram, Day #1
- LESSON 5: Free Body Diagrams, Day #2
- LESSON 6: Mass vs. Weight
- LESSON 7: Newton's 3rd Law, Day 1
- LESSON 8: Newton's 3rd Law, Day 2
- LESSON 9: Unit 4 Test: Inertia and Interactions