Newton's First Law
Lesson 4 of 16
Objective: Students will be able to define and apply Newton's First Law.
In preparation for our inertia in action activity, I organize and lay out one cup, one index card, several pennies, 2 straws, and a marble for each group. Having these materials ready to go before class allows the lesson to flow without interruption.
So far in this unit, students have defined force, looked at the vector nature of forces, and drawn free body diagrams. After I define Newton's First Law for students (in anticipation of covering Newton's Second Law (HS-PS2-1)), students see inertia in action (SP 5 & 8). Today's goal is to connect our definition of force with the concept of inertia, so class will end with students reflecting in pairs.
It's time for students to take out their science notebooks and get ready to learn the fundamentals of Newton's First Law. Today, I define the first law, discuss what this means in terms of constant velocity, and describe the relationship between mass and inertia. 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, but we've practiced it quite a bit throughout earlier lessons in the course as well.
I display my Newton's First Law presentation (also available in PPT) to help the students understand what they need to write down, but I also have a hard copy of the document in my hand. The printed copy includes notes (viewable only when the file is downloaded) that I've written to myself to make sure I accomplish the learning goals as we work through the presentation. The nice thing about having a hard copy in my hand is that if I think of a great side-note or new example, I have the ability to write it down immediately. Having this hard copy also allows me to walk throughout the room as I ask students questions. For example, in the fourth slide, I ask students "What is the difference between a zero net force and no forces acting on an object?" As we go through the notes, I welcome questions and expect participation from the students.
The presentation ends with a sample problem, so I make sure I use appropriate wait time for students to read the problem and think about their answers. This process usually takes about two minutes, but then I ask students to be able to explain their answers. The explanation is the part that often stumps them, so I wait another minute or two before asking for student volunteers to share their ideas. Once we've successfully reached the answer through the discussion, I reveal the answers on the slide (each one will appear in red with a mouse click) and it's time to move into the next activity.
An Inertia Activity
During the previous class I assigned partners, so today I allow the students freedom in choosing lab partners. Groups of two to four seem to work best so that all students have an opportunity to be engaged in the activity. After they've chosen their groups, someone from each group needs to get a copy of the activity and the materials from the front of the room.
The procedure in the lab document is straight-forward, but I still make sure to circulate throughout the room and check-in with the groups. The goal of this activity is to show students that all objects have inertia and act accordingly to Newton's First Law. When students place a penny on a notecard and then flick that notecard away, they see the penny sink into the cup. During the marble activity, students use straws to move a marble across the table. Both activities are meant to show students that an object resists a change in its motion. In the case of the penny, the penny falls into the cup because its inertia held it in its location horizontally while a force (gravity) changed its vertical motion once the the index card was removed. In the case of the marble, the marble is difficult to stop rolling because its inertia carries it forward and the blown air cannot provide much force to stop it.
I walk around the room offering encouragement as the students flick the index card and blow the marbles, and ask questions such as "Why does the penny fall into the cup?" or "How is Newton's First Law visible in the marble?" Students understand the expectation that once they have completed collecting the data it's time to move into the analysis section. In fact, it's this analysis section and writing that earns students points for this activity. While the students do find it fun to see inertia in action, the purpose of this lab is for students to explain what they see in terms of Newton's First Law.
When there is approximately 10 minutes prior to the end of class (5 minutes left of the time I've allowed for this activity), I ask students to put everything back the way they found it and return to their seats. I also tell them at this point that the lab is due at the start of the next class meeting.
The closure for today starts by asking students to sit quietly and think about one word that describes the inertia activity. They get about a minute to quietly reflect, and then students turn to someone near them and share their word. I'm not overly strict on if students share with just one person or if they share in small groups, because I want everyone to be given an opportunity to participate. The goal of this closure is to provide students with an opportunity to debrief the lab activity and its application of Newton's First Law.
I walk throughout the room during the quiet reflection time to ensure my students are focused, but I also walk as students are sharing with their neighbors. I ask students to share their words with me, so that I can check their level of understanding and reflect on my teaching practices. When I heard words such as "free body diagram," "vector," and "net force" I was confident that my students had grasped the content of today's lesson.