Torque on a Stick

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Students will be able to calculate torque on a meter stick under a variety of situations.

Big Idea

Students hang weights from a meter stick and calculate the resulting torque.

Context & Equipment Needs

During the previous lesson, students defined torque and worked through several examples of solving torque problems. The goal for today's lesson is then to apply that knowledge of torque in a lab activity. Specifically, students calculate clockwise and counter-clockwise torques (SP5 & 8) for balanced weight and unbalanced weight situations. I start the class with a ranking activity before moving into the actual lab activity. Today's lesson ends with a fish bowl closure.

This lab requires the following equipment for each lab group: 1 ring stand, a few masses, 1 piece of string, and a meter stick.

Introduction with a Ranking Task

10 minutes

This introduction is meant to refresh students' thinking and help them assess their individual level of understanding about torque. I choose this specific task because it asks students to rank situations in order of greatest to least torque. The task also requires students to demonstrate an understanding tension as it involves masses hanging from disks. These are all concepts that will need to be applied in today's lab activity or on the AP Physics 1 exam, so it gives students a nice opportunity to activate prior knowledge. I always have any introductory activity ready to go when students walk into my classroom to help with time management, so today's ranking task is already projected onto a screen at the front of the room.

Once the students are settled, I read the instructions to the activity. My reading of the instructions is to ensure students understand that class has started! After I've finished, I emphasize to students that they should work individually and take about 5 minutes to rank the torques, explain their reasoning, and then assess their level of confidence. During these 5 minutes of work time, I walk around the room and informally assess how students are doing with simple glances at their work. My changes in location help students stay quiet and focused.  

When the 5 minutes are over, I am back at the front of the room and reveal the answers by writing them onto the front screen: F, B, A, D, E, C. I always start by asking if anyone got all of the torques in the correct order. Since my students are fabulous, inevitably there is always at least one who did and is willing to admit it. It's that student that I then ask to explain why they ranked the situations the way they did. If the explanation is complete and clear, I commend the student! In the event the explanation needs to be expanded (as the student in this sample used incorrect thinking and subtracted the masses) I still commend the student, but then contribute any of that missing information so the whole class has a complete understanding. I end the introductory activity by asking if there are any questions and then collecting the ranking tasks.

Torque on a Stick Activity

35 minutes

Since the concept of torque is now fresh in the students' minds after our ranking task introduction, it's time to move into the lab activity. I know that my students cannot finish the lab write-up in class today, so I allow them to choose two partners. Their choosing of partners ensures each student feels comfortable meeting outside of class. Also, my students are mature and have a good rapport with each other, so I never have to worry about someone being left out of a grouping. In the event that someone is absent, I do make an exception for one group of four. After they have chosen their groups, they come to the front of the room to grab a lab sheet and the equipment, then move toward an open lab station.  

I announce to students that they should go right to the first page and start. They are already familiar with the expectation that they need to check their lab stations with the materials list to ensure they have everything needed (a meter stick, a ring stand, several masses, and a piece of string) for the lab. It is my rule that if something is missing at the end of the class that group is charged with the cost of the missing item. I find doing this holds kids accountable and ensures my materials don't fall into someone's pocket.  

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. Students start by balancing the meter stick at its center of mass by hanging a string from a ring stand. At this point, the total torque is zero because the meter stick is balanced. Then, students hang 2 different masses on each side of the balancing point, record their locations, and calculate the torque on the stick. Students repeat the process after adding a third weight. The second part of the lab has students create a new pivot point of their choice. Then, students repeat the process with adding one mass and then adding a second mass to balance the stick.

When there is approximately ten minutes prior to the end of class (five 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. If students have time left after they finished collecting data, they are aware of the expectation to start on their calculations. About 50% of my students have a few minutes to start on their calculations, while the other half of the class takes the entire time allotted to collect data. I also tell students when the lab is due (I usually give them a full week).

Fish Bowl Closure

5 minutes

Students are back at their desks after cleaning their lab stations, so now I pass around small slips of paper (about the size of a standard sticky note). I use a stack of blank paper and chop it up with a paper cutter in the faculty lounge before class begins. Each student takes at least one of these slips of paper so they can participate in our fishbowl closure activity. I explain to my students that they need to write down one question that they'd like answered about the general concept of torque or specifically about the lab. Each student must write something down, fold the slip of paper in half, and place it in the clear bowl at the front of the room.

I keep the room pretty quiet for this closure, as I want students to really reflect on what information they are feeling unsure about. I also don't want students to all have the same question, which is more likely to happen if students are able to share. In the past I've noticed that students are sometimes too shy to ask questions, so this activity should give them confidence by being anonymous. And, I pass around the slips of paper instead of simply handing a student just one so that if they choose, a student can take more than one slip for multiple questions.  

This is a pretty fun activity and I read and answer the questions during the next class period. I use an actual fish bowl to make the name fit the activity, so that's empty and sitting on my desk at the start of class. And, to ensure all questions are appropriate and meaningful such as these, I remind students of these requirements and read each slip of paper before going over them!

Generally, I've noticed that students ask questions that might make them look silly in front of their peers. For example, one student wrote a question asking what would be considered an appropriate value for percent error. This particular student is often competing with his peers to get the best data, so I'm guessing he's too embarrassed to come right out and ask the question. After I read each of their questions, I make notes on the lab so that when I use this resource again I know what adjustments need to be made. Also, if I notice any major themes of content that students are struggling with, I can plan a remediation activity for the next class meeting.