2. Heavy for Size:  Investigation 2.2

What makes a good weight line?

3. Develop rules for weight lines

Discussion 15 Mins Notebook

Purpose of the discussion

The purpose of the discussion is for students to consolidate their ideas about what makes a good weight line and to recognize the importance of consistent measurement. Return to the investigation question for the discussion.

Engage students in the focus question

Students’ initial discussion of the concept cartoon sets the stage for consolidating ideas about the investigation question:

What makes a good weight line?

Ask students to share their thoughts about the four weight lines in the concept cartoon, and then pose a challenge:

Could we make a list of rules for good weight lines?

Help specify students’ thinking. Why do they think this is a good rule? What would happen if the rule was not followed? In particular, help students understand that a good weight line will accurately represent the "closeness" or "distance" that the weights have each to each. That way our eyes can make sense of their relative weights. In other words, a good weight line will show how much more one thing weighs than another.

As the class agrees on some rules, record them on the list you prepared before class and have students copy them into their notebooks [Rules for good weight lines].

Some rules for good weight lines:

• The line should start at zero.
• The line should end at a value higher than the heaviest object that will be placed on it, but not much higher; this is called giving an appropriate range.
• The unit of measure should be clearly indicated.
• The line should include some weight marks showing some values between zero and the end value.
• The weight marks should be evenly spaced, and the distance between marks should represent the same amount of weight each time; the distance between the marks is called the interval.
• The interval doesn't have to be 1 unit; it could be any regular increase, e.g., 2g or 10oz or 50lb — whatever best suits the purpose of the line.
• The line should be long enough to show the information clearly.

Summarize the Discussion

Leave them with this thought:

Now we have some rules for good weight lines. Do you think you can make a good weight line that can show how the weights of sand, soil, water, and mineral oil compare — that can show how much heavier one is than another? In what ways would your weight lines be the same?

Save the class list for the next investigation.