Tension Lab

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Students will be able to calculate tensions and sum forces.

Big Idea

Students explore tensions with spring scales and different masses.

Context & Equipment Needs

During the previous lesson, students defined tension and worked through several examples of solving tension problems. The goal for today's lesson is then to apply that knowledge of forces (HS-PS2-1) in a lab activity. Specifically, students will calculate tension (SP8) after changing string lengths, masses, and string angles (SP3). I start the class with a ranking activity before moving into the actual lab activity. Today's lesson ends with a fish bowl closure activity.

This lab requires the following equipment for each lab group: 2 ring stands, 2 spring scales, 2 pieces of string, several small masses, a meter stick, and a protractor.


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 tension. I choose this specific task because it asks students to rank situations in order of greatest to least tension. The task also requires students to demonstrate an understanding of pulleys and tension as it relates to different masses and velocities. These are all concepts that will need to be applied in today's lab activity, 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 tensions, 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: A, B=E, C, D=F. I always start by asking if anyone got all of the tensions in the correct order.  Since my students are rock stars, 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 graphs the way they did. If the explanation is complete and clear, I commend the student! In the event the explanation needs to be expanded, 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. 

Tension Lab Activity

35 minutes

Because I know that my students will not finish the lab write-up in class today, I allow them to choose two partners so that they feel comfortable meeting with the person 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 need to come to the front of the room to grab a lab and the equipment, then move toward an open lab station.  

I announce to students that they should go right to the first page and get started. They are already familiar with the expectation that they need to check their lab stations with the materials list to have everything needed 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. After students hang a mass from two strings (each attached to a spring scale - see lab handout for an illustration), they must maneuver the ring stands to create one of the identified angles. Then, students compare the readings on the spring scales to the weight of the hanging mass before adjusting the apparatus to a new angle. The second part of the lab adds a meter stick to the apparatus so students can calculate tensions of two different angles (see lab handout for this illustration as well).

If time allows and students have the desire, I let them try to break the string with their mass. 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 when the lab will be due (I usually give them a full week) and remind them to reference the guidelines for a formal lab report.

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 colored paper and chop it up with a paper cutter in the faculty lounge before class begins. Each student should take 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 tension 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 about how to calculate percent difference, which is something she should definitely know by now. However, this type of question is not silly at all and it's perfectly understandable that she may have forgotten how to calculate percent error if she hasn't done it recently. 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.