Purpose of the discussion

The purpose of the discussion is to consolidate understanding that the gram is the standard unit for measuring weight and that weight can be represented on a weight line.

Set the stage for the discussion by reviewing the weight line. Check students’ understanding of using “distance from zero” to represent weight. Confirm that they understand that the weight line is continuous, i.e., that objects can fall anywhere on the line — either very close together or very far apart — and all still be part of the same line.

Engage students in the focus question

Ask the focus question:

How can grams help us compare weights?

Ask the class what they think about grams as a unit of measure.

• Are grams easier to use than paper clips or counting bears?
• Were you able to estimate gram weights accurately?
• Would pounds work as well as grams for weighing the objects? Why or why not?
• What are the advantages of grams if we want to compare weights?

Finally, return students’ attention to the weight line and start a discussion about the relationship between size and weight.

• Are there objects that weigh about the same but have different sizes? Can anyone explain how this can happen?
• Are there objects here that are just 1 gram different in weight? Can objects be closer than 1 gram in weight?
• What are the lightest objects on the line? How many grams do they weigh? How do the sizes of the light objects compare? Are there any surprises?
• What are the heaviest objects on the line? How many grams do they weigh? How do the sizes of the heavy objects compare? Any surprises?

Students should now be describing their observations in terms of grams. They should begin to puzzle about the relationship between size and weight.

Provide time for students to reflect and write responses in the Here’s what I’m thinking now section of the Notebook page.