Learning Goal: To identify the function of a triple beam balance and to use it properly to measure mass
Language Goal: To explain how mass is different than weight
A triple beam balance (TBB) is the tool we use to measure mass in science class. It's what's required for our state science exams therefore, we use them in our experiments, as well.
Before class begins, I place a triple beam balance on every table (students sit in groups of 3, but you can distribute them however you need to).
I have the students respond to the following prompts on their notes sheet:
Describe what the object on your table looks like.
Describe what the object might do.
Explain how or why you may use this object.
After 3-4 minutes of writing and responding, I will ask the students to share out a few of their responses. (I generally do this as a whole class discussion in order to spark interest for those who may not know anything about the TBB.)
This lesson provides students with a chance to create their own meaning of a concept (mass) and construct their own definition of what it is. The standard SP7 is addressed as the students are writing their own claim for what mass is and supporting it with evidence from the lesson. They are also working on SP5 by practicing their measuring and analytic skills.
Next, I ask the kids to work in their small groups (you can have them work individually, as well). I put this picture on the board and have them answer the following prompt on their notes sheet or in their notebooks:
What do you notice about the MASS of the astronaut on earth and the moon?
What do you notice about the WEIGHT of the astronaut on earth and the moon?
What is a possible explanation (reason) the astronaut WEIGHS LESS on the moon than on earth?
Explain how MASS and WEIGHT are different.
After about 4-5 minutes, I ask the groups to share their explanations for the differences in weight but not in mass. I then ask them to attempt to explain how mass and weight are different. (At this point, usually at least one student will mention gravity and how there's less of it on the moon. I then ask them to consider what gravity might have to do with weight. If no one mentions gravity, I will say, "Who has ever seen video footage of astronauts walking on the moon? Do they bounce? Does it seem like they are floating? Why is that?" This will spark a conversation about gravity.)
After the students have thought about how mass of the astronaut doesn't change but the weight does, I show them a NASA video. This video is narrated by two astronauts who discuss the differences between weight and mass. As the students are watching, I have the kids complete the notes chart in the EXPLAIN section of their graphic organizer to help them jot down ideas during the video.
I stop at the following points and ask the following questions:
1:25 On which object does the astronaut weigh the least? The most? Which has the most gravity? As gravity increases, so does...?
2:02 Mass is the measurement of how much matter, or STUFF, makes up the object. Everything on earth is made of matter. Think about this: we said the astronaut's mass stayed the same, but his weight did not. Can someone explain this?
2:49 Stop the video.
Now that everyone has a general understanding of the difference between weight and mass, I explain that when we use the triple beam balance (TBB), we are measuring mass, not weight. For the next 10 minutes, I have the students following the procedure for using the TBB on their notes sheet. They will measure the mass of 1 object while working together as a group. One person can read the procedure. The other can measure. All will write down the measurements.
Note: It will be obvious during this time, who can and cannot follow a procedure. Make sure to place one strong student in that group who you know will keep the others on task and following the procedure in the proper order. Circle around to all the groups making sure they are using the scale properly. I give all groups the same object so that I can assess their usage of the TBB by checking their measurements, as well.
Following the same procedure in the notes sheet, students will continue to work together measuring the mass of the remaining 4 objects. They will take turns, each person using the TBB at least one time and calculating the measurement. All students are responsible for writing down the measurements on their notes sheets.
Note: I circle around at this time and make sure everyone is on task and staying true to the procedure, helping students to refer back if they are unsure of the next step.
When students finish, they will answer the following question in their notes sheet:
An astronaut's weight changes when on the surface of the moon. The astronaut's mass does not change. Explain why the weight changes and the mass does not.
Student Work Samples and Evaluations:
Average Student Work Sample: This student was able to discuss the fact that the moon has less gravity and mention that it affects mass, but wasn't able to discuss how gravity directly affects mass.
Exemplary Student Work Sample: This student mentions the fact that because the moon has less gravity, the astronaut's weight is less. He also mentions that the mass does not change because the astronaut is still the same size.