Tensions, Pulleys, & Elevators - Oh My!

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Students will apply their knowledge of Newton's Second Law when calculating tensions in pulley and elevator systems.

Big Idea

Students stretch, pull, and lift their way into the world of forces!


At this point in the course, students have a solid understanding of Newton's Second Law. In yesterday's lesson students applied Newton's Second Law to ramps, and today's goal is to apply the Second Law to situations that involve tension, pulleys, and elevators (HS-PS2-1). I try to offer a variety of strategies to accomplish our goal, so the lesson starts with a short video and then goes into a paired reading activity (SP8). Finally, students get to apply their new knowledge towards the end of class with collaborative problem solving.

Elevator Free Fall Video Introduction

5 minutes

Sometimes I like to get out of the routine of having a problem or demo as the introduction, so today I choose to show an interesting video of an elevator in free fall. The video is about a minute long, which means it is just long enough to pique student interest and get them focused on today's goal: to learn how Newton's Second Law can be applied in tension, pulley, and elevator problems. I do like to get started right when the bell rings however, so I have the video loaded and displayed on the front board when the students enter the room.

After the video is over, I assign each student a letter of the alphabet. I usually choose the person closest to me and give her A, then the next person B, and so on. Students then have about a minute to think of a word that describes the video that begins with their assigned letter. After they've been given a minute to think, I point to the student who had A to share her word and why she chose that word. Then, we go around the room until everyone has shared. In the past, if a student has struggled we pause and think of a word as a class to help that person out.  

This word list is an example of what students shared. It gives me the satisfaction of knowing that I captured their attention and insight into what students remember from our unit on free fall. Not only does keeping a copy of these word lists remind me of how much fun this little introduction is, but it also helps me set an instructional pace.



Paired Reading Activity

30 minutes

To learn how Newton's Second Law is applied to tensions, pulleys & elevators, students engage in a paired reading activity.  I introduce this activity by passing out the paired reading document to each student and explaining that I have already chosen their partners.  Partners work best for this activity, and I already made a list of who will be working together based on their current grade in the class. I don't tell the students how I've paired them, but I ranked the class by overall grade, split the list in half, and then matched the first names on each list. Because there was an odd number of students, I made an exception and had one group of three. Pairing students this way forces them to work with someone different and ensures that ability levels are somewhat equal. To save class time, I printed this list and organized the pairs prior to class starting.

Now that students understand they don't need to scramble for a partner and have the document in front of them, I share how this paired reading activity works. I expect that the students read one page at a time individually while annotating the text with information they deem important. Students should stop reading when they get to the end of the page. Once both partners have come to the end of page 1, they should exchange their annotations and copy down onto their document any ideas that they didn't originally have. For example, if one student thinks the definition of tension is important and her partner didn't identify that, her partner needs to write that down on her own paper. The students then repeat this process until they have gone through all seven pages of the document. They have approximately 30 minutes to work through this packet, so they should be reading and discussing each page every 3 or 4 minutes. 

I share with students that this activity has three purposes. The first is to practice reading detailed information in a short amount of time. The AP Physics 1 exam has been redesigned to include more reading, so I want students to be able to practice reading and pulling out important information under a time constraint. This parallels the second goal, which is to build students' stamina for reading physics material. At first glance the reading might seem a bit long, but it was intentionally selected to help students students prepare for the 3 hours they will spend taking the AP exam. The final goal is that students must grasp an understanding of the concepts, vocabulary, and equations used in the Second Law applications.

After I'm done giving the instructions, I reveal the pairs by simply reading them from the organized list.  I have students move so they are sitting with each other, but since they are AP students I let them organize themselves and choose their own seat locations.  Once everyone is settled I put the end time of the activity on the front board and begin to circulate the room.  My circulation lets me know if students are on task and allows me to redirect students if I hear misconceptions or off-task conversations.

Small Group Problem Solving

15 minutes

As closure and an informal assessment, students have the rest of the class to start tonight's homework. I call it informal because I don't want students to get nervous that it counts as a quiz or test grade. Since my goal is to assess their level of applying Newton's Second Law, I collect and grade the 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. Students should get through half the homework before they leave class today, if they use the class time wisely and have well-organized notes. Ideally, the homework assignment should look like this when it is turned in at the start of the next class period.

Students remain with their partners from the paired reading activity as I pass out a copy of the homework to each student. This is an assignment that needs to be completed by each individual, although they may use their partner as a resource while working in class. I encourage collaboration throughout their work time and since students are paired based on ability the "weaker" student is able to use the "stronger" student as a resource. It is a goal of this strategy that the "stronger" student improves his or her knowledge by helping the "weaker" student work through the problems. Also, I walk around to offer help or problem solve with the students as they are working. You might hear me say "Well, which equation does your partner think you should use?" in response to a student asking about an equation. Or, I might say "Look back at the example on page 4, there's some very important information in there." My style is to lead the students to the answer, not just provide it for them. That being said, if a student is continually struggling and in obvious need of being shown the answer, I accommodate him or her.  

This is our closure activity for today and it's meant to have students apply their newly learned knowledge from the paired reading activity. At an absolute minimum students should have successfully answered the first question and been working on the second question. Many students were at this point or farther, so I know the paired reading activity and collaborative work times were both a success. These two activities combined are my attempt to take a step towards a flipped classroom. I like students to have me as a resource when they work through problems, and I think it helps them build confidence. 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.