Talking About Torque

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Students will be able to define torque qualitatively and quantitatively and identify its relationship to angular mechanics.

Big Idea

Why are door knobs located opposite of hinges on a door? Today, students answer this question and explore torque.

Context & Equipment Needs

As our first lesson of this Rotational Motion unit, today's goal is to define torque and demonstrate situations in which torque is present in students' daily lives. Defining torque allows students to then broaden their physics knowledge to include definitions of moment of inertia, angular momentum, and rotational kinetic energy. Students participate in a demo of torque before watching and taking notes via a meaningful video from the Khan Academy (SP8). The lesson closes with students applying their new knowledge in a collaborative problem solving activity (SP5).

For the demonstration, each student (or pair of students) needs a meter stick, roll of tape, and spring scale.

Demonstrating Torque

10 minutes

As students enter the room, I ask that they each grab a meter stick, a roll of masking tape, and spring scale from the front of the room for today's introduction to torque. I have the materials in neat piles, near the door, so students can each take one and we can start class right when the bell rings. 

When the bell rings, I instruct students to tape one end of the meter stick down on their tables and attach the spring scale to the other end. I'm showing students how to set this up as I'm telling them that the point of the demo is to explore how force varies. Specifically, I want students to move the spring scale incrementally towards the tape and observe how the force changes. If done properly, the force needed to lift the meter stick increases as the spring scale gets closer to the tape.

After students have had about five minutes to explore, they return the equipment to the front of the room, sit down, and we debrief about their experiences. I use a discussion guide to remind me of certain questions I want to ask, with a goal of getting students to realize that distance from a pivot point impacts the required force. The guide includes notes in purple that I took during the actual discussion. I record these notes so I can refer back to them throughout the unit and address any topics that need to be clarified.

Defining Torque Video Activity

20 minutes

Because there are so many great resources out there and I had an introduction that included a teacher-led discussion, I've decided to use this Khan Academy video to present the definition of torque. Before I start the video I make sure my expectations are clear. Students need to be sitting quietly, listening and watching the video, and taking notes on meaningful material. To me, meaningful material includes any reference to previously learned concepts, equations, vocabulary, and examples. I am telling my students these expectations as I'm on my way to start the video.

As we watch the video I pause it in several places to give my students an opportunity to process and internalize this new information. I use a discussion guide to remind me where to stop and what important material or questions I want to share with the students. When I pause the video I'm somewhat flexible in our discussions, meaning if a student has questions or needs further explanation I take the time to do that. I always write down any equations on the front board so that students are sure they have copied them properly.

The goal of showing this video extends far beyond defining torque. Students should have a complete set of notes from which to study that includes several examples. Also, the end of the video discusses how doubling the distance from a pivot point cuts in half the force required to rotate the object. This concept is what students explored and discussed in the introductory demo, so I make sure to refer to the demo in the final part of our video discussion.

Collaboratively Solving Torque Problems

20 minutes

As closure and an informal summative assessment, students have the rest of the class to start their torque 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 understanding, 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 check the pacing of the course and make sure my students are ready to move on to the next lesson.

Students remain at their lab tables as I pass out a copy of the homework to each student. This is an assignment that needs to be completed by each student on a separate sheet of paper (like this), although they may use their table-mates as a resource while working in class. I encourage collaboration and hope students help each other solve as many of these problems as possible before leaving the classroom. These problems are specifically chosen because they cover the torque equation in a straight-forward manner. Eventually the difficulty of these problems will increase, but for now my goal is to ensure that students understand the fundamental concept of torque. Since our introductory activity ran a little long, most students got through question two before the class time was over.

As students work, I walk around to offer help, make sure students are on task, and problem solve with the students. For example, when I walk around and hear students talking about bird nests (discussed in the first question), I know they are on task. My style is to lead the students towards the answers, not just provide them with the correct answer. That being said, if a student is continually struggling and is 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 video activity. I am also trying 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. 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.