Teacher resources and professional development across the curriculum

Teacher professional development and classroom resources across the curriculum

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Teaching Math: A Video Library, 5-8

Building Rafts With Rods

Video Overview

Michelle Mullin, demonstrating on the overhead, explains that each group will calculate the surface area and volume of a "raft" of n rods, where n goes from 1 to 10.

After that, they have three tasks they can do in any order: graph their data, make formulas for surface area and volume, and write a question that will introduce the task to another class.

The groups work for some time on their tasks. Ms. Mullin circulates, helping groups with problems and checking in with them.

At the end of the class, she has a few groups make brief presentations showing how far they've gotten.

An Exploration for Teacher Workshops

Materials: grid paper, cubes

Work in small groups to make the structures below. Then discuss: what does the fourth structure look like? The seventh?

(Structure #1)

(Structure #2)

(Structure #3)

Work in small groups to calculate the surface area and volume of each structure. Look for a pattern. Check your conjecture on the fourth and seventh structures in the series. Generalize the pattern into a formula so that you can predict the surface area and volume for any structure in the series, no matter how large.

Some Questions

  1. What was the hardest part of figuring out the volume or surface area? Why?
  2. How did you record your information? How did you figure out the formula? How did you record your formula?
  3. Why are you sure your formula works for all structures in the series?
  4. Suppose you were to graph surface area against volume. What shape graph would you get (straight, curved, wiggly)?
  5. Does a task like this legitimately belong in pre-algebra? Why or why not? (When you watch the video, look to see how the students' task differs from this one.)


  1. Think about this: Volume is supposed to be cubic. Why is volume quadratic here?
  2. If you plot surface area against volume (instead of against n), the graph is almost but not quite straight. Why is the surface area vs. volume graph straight in the video? Why isn't straight here?

Additional Discussion Topics

Here are some additional ideas for discussion that arise in the video:


Discussion Questions

These questions appear at the end of the video. Here are some follow-up ideas and prompts to help get a discussion going.

From what you've just seen, what do you know about these students' mathematical power?
Think about the student discourse you heard in the video. What evidence did you see or hear of understanding? What evidence of confusion? What can you tell from their graphs? What more might you ask students in order to learn how they are approaching this task?

How can you help students learn to generalize patterns into formulas?
What did Ms. Mullin do to help students figure out the formulas? What else could she have done? What do you do when students are struggling to figure out a formula?

What are the components of a good task?
In what ways is the task in the video better for this class than the one in the Exploration?

Consider changing this task and imagine how it might go differently. For example, what if the task were more structured? What if it were more open-ended? What if the situation generated nonlinear functions instead of linear ones? What if (as in the video The Location) students had used plain paper instead of grid paper for their posters?

In those cases, what else might students learn? What might they be confused about?


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