As a whole group, introduce the idea of body measurements. This can be done in ONE of the following ways:
- Quickly review one the two other Illuminations lessons, Sizing Up or How Long? How Wide? How Tall? How Deep?,
with questions such as "What did you notice about the measurements of
(your arm) and (your leg)?" Be sure to pick two measurements that are
equal or very close. Ask, "Do you think this is always true for every
- Give a simple art lesson (or invite an art teacher) on how to draw a human body that is in proportion.
- Visit goldennumber.net.
This web site has color-coded lines of measurement for the human body,
face, and hand that provide a nice visual description of the
mathematical task of this lesson.
The purpose of this introduction is to grab student interest in
doing an investigation of body measurement ratios, so pick the "hook"
that best fits your class and your skills. The introduction should only
take 5‑10 minutes.
Then tell the students that they will be taking lots of
different measurements to find out whether lengths and/or
circumferences of body parts such as arms, legs, and necks are just
random lengths or if they are related somehow. They will work
independently but will sometimes need to request help from a partner.
The measurements will be written on the Student Recording Sheet.
Before students get started, explain the My Measure
activity sheet and model how to take the measurements accurately. Give
special attention to the circumference measures, as many students may
not be familiar with this. The teacher should model one of the tasks
before students get started.
Thinking out loud as you show an example will help students
understand what to do. For instance, say, "Okay, I’m going to put the
end of the measuring strip right here, at my wrist. Now, I’m going to
lay the strip in a straight path along my forearm. I’m going to fold
the paper strip at the end of my forearm so I know where to cut the
strip. There, I cut the strip. Now I’ll measure it with a centimeter
ruler. Let’s see, it’s about 27 cm. I’ll record that data on the
- Use the paper strips to measure a body part. Cut the strip to fit
as accurately as possible. (Measurements are never absolutely exact.)
Paper strips can be taped together for longer measurements. Tape first
and then use the longer strip for measuring; this avoids the problem of
errors caused by overlapping strips.
- Use the ruler to measure the paper strip. Label the strip with the body part and the measurement (for later use).
- Record the measurement on the My Measure activity sheet.
To aid with classroom management, consider these suggestions.
- There are a lot of measurements to be taken. To save class time,
have students get started during class and then complete their
measurements as homework; or divide the class into three groups and
assign the questions on the My Measure activity sheet as follows: Group 1, Questions 1‑3; Group 2, Questions 4‑5; Group 3, Questions 6‑7.
- For measuring faces, consider using close-up photos of the
students. These will need to be taken and developed in advance of the
lesson. Students can measure the photos rather than their faces.
Allow the students to get started and work for about 5-10
minutes. At that point, pull the class together to discuss what is
going well and what problems the students are encountering. It’s better
to address these concerns before too much class time has passed. This
is also a good time to demonstrate the following relationships, which
the students need to understand for the analysis portion of the lesson:
- "equal" – Show two paper strips, A and B. Have the students say, "A equals B in length."
- "half" – Show two paper strips, A and B. Fold B in half, cut
on the fold line. Compare the whole strip (A) to the half strip (the
new B). Have the students say, "A is twice the length of B" and "B is
half the length of A."
- "one and a half times" – Show three paper strips, A, B,
and C. Fold C in half, cut on the fold line. Tape C (half) to B.
Compare B‑C to A.
Have the students say, "B‑C is the same and half again as long as A"
and "B‑C is one and a half times as long as A."
The teacher should model how this information applies in the actual
measurement tasks. She could measure her forearm and upper arm, find
that the measurements are "close enough" to conclude that they are
roughly equal in length. That is, measurements of 26 cm and 27 cm are
roughly equal since measurements are always a little imprecise.