Practicing Newton's Second Law

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Students will be able to apply Newton's Second Law to a variety of situations.

Big Idea

So many forces, so little time! Today, students collaboratively solve problems using Newton's Second Law.


Students have become familiar with Newton's Second Law in 1-dimension and 2-dimensions, and have applied their knowledge in a simulation activity, so today students practice quantitatively applying Newton's Second Law (HS-PS2-1). To accomplish our goal, I start with a misconception check before students work through practice problems involving several different situations, including tension and pulley systems. The problem set not only requires that students arrive at the correct answer, but must also justify answers (SP5 & SP7). Finally, students demonstrate their success solving Second Law problems with a "pick a problem" quiz.

Misconception Check

5 minutes

When students walk into the classroom, I have a misconception written on the board at the front of the room. Today's misconception reads "Forces cause motion." and its purpose is to get students thinking about Newton's Second Law. Intentionally, there is no writing during this activity because I don't want to risk students copying down the misconception and then studying from it in the future.

I ask students to clear their desks and sit quietly for the activity because I really want them to think about what they know and how it applies to the statement on the board. When the students are ready, I read the statement and then ask students if they agree or disagree. I also remind them that in the world of AP Physics, an answer is not enough - we must justify everything. Then, I stay at the front of the room and just leave students to think for 2 or 3 minutes. 

After time is up, I ask student to raise their hands if they agree with the statement written on the board. I take a mental note of these students before asking one of them to explain their reasoning. Then, I ask a student who did not have his hand up to share why he disagreed with the statement. This process continues until a several students from both sides have shared their justifications. As an example, one student shared that "without a force all objects would remain stationary," and a second student immediately responded by asking that student how he explains things that move with constant velocity.

Before moving into our final activity, I ensure that students understand that this is a BIG misconception by telling them so in an over-the-top, flamboyant, quirky manner. I wave my arms as I tell them that "Forces do NOT cause motion." Then, I say "forces cause acceleration," and I justify that statement with an example of how an object can be moving with constant velocity. As we move into the next part of the class, I make a special effort to check-in with students that agreed with the misconception to see how they are feeling about things.

Collaborative Problem Solving

40 minutes

Students are given about 35 minutes to work on today's summative assessment: second law practice problems. The assessment is summative as it includes use of the kinematic equations that were learned earlier in the school year. Since my goal is to assess their level of understanding and use of prior knowledge, I will collect and grade one problem at the end of class and the entire assignment for accuracy at the start of the next class meeting. Not only do I want to give students personalized feedback on this homework assignment, I also want to check the pacing of the course and make sure my students are ready to move on to the next lesson.

When I collect an assignment and grade it for accuracy, I'm looking for much more than correct answers with units. I expect that students include all necessary equations and show the corresponding substitutions for each problem. The AP Physics 1 exam emphasizes justification of answers and awards points for showing work, so I try to grade my assignments in a similar manner. For example, the first problem was graded out of 6 points: 1 point each for the correct equations, 1 point for the right substitutions, 1 point each for the correct answers, and 1 point for the proper units.

Students choose to work with one other student as I pass out a copy of the problem set to each student. This is an assignment that needs to be completed by each student, although each student may use his or her partner as a resource. I encourage collaboration throughout their work time and I walk around to offer help or problem solve with the students as they are working. These problems represent the concepts we covered in the last few classes, such as the acceleration of masses in a pulley system, tension found in cables, and forces involved in ramp situations. I choose these problems because they are similar to problems that have been found on past AP Physics exams and are also the types of problems students will find on the unit test.

This activity is meant to give students an opportunity to practice application with peer and teacher support. My support method is to lead the students to the answer, not just provide it for them. I work to accommodate the individual student as I walk around by making the material meaningful or relevant. I also strive towards developing and enhancing students' conceptual understanding. I like students to have me and their peers as a resource when they work through problems, as I think it helps them build confidence. In the past I've attempted to do entire class periods of a full flipped classroom, but it's hard to hold the students accountable for digesting the needed material.  I find that a combination of work time (that lasts right up until the bell rings) and in-class learning best fits the needs of my students.

Pick a Problem Quiz Closure

5 minutes

Today's closure forces students to evaluate their level of confidence on applying Newton's Second Law. With about 5 minutes left in class, I notify students that they must select one of their completed problems to be graded. They make this selection by circling the problem, and that problem is specifically graded as if it were on a quiz. To me and my students, this means that they must include the equation, substitution of values, and final answer with units. 

I've avoided telling students this information until the last moments of class. In the past, students have spent far too long concentrating on only one problem and not finishing the assignment if they knew ahead of time about the pick a problem quiz. I wanted students to work through the entire assignment because each problem is a valuable opportunity to review and apply concepts. Also, I grade the entire assignment to offer individualized feedback, so it's crucial that students are motivated to do the entire assignment.

After students are aware that they must choose 1 problem to be counted towards a quiz grade, I give them a few minutes to choose the problem and make any necessary adjustments. They still have the ability to collaborate throughout this time, so I encourage students to proof each other's work. However, time is up when the bell rings and students must hand in their completed problem sets with their chosen quiz problem as they leave the room.