# Mathematical Modeling Lesson Plan 1: Mathematical Modeling, Circular Movement and Transmission Ratios Overview:
In this lesson, students will learn to use mathematical models to represent real life situations. In particular, they will use tables and equations to represent the relationship between the number of revolutions made by a “driver” and a “follower” (two connected gears in a system), and they will explain the significance of the radii of the gears in regard to this relationship.

Time Allotment:
Two 50-minute class periods

Subject Matter:
Transmission factors, explicit and recursive equations, and mathematical modeling

Learning Objectives:
Students will be able to:

• Understand the significance of the transmission factor in the design of rotating objects (gears) that are connected.
• Determine the transmission factor using the radii of connected pulleys.
• Describe the differences between negative and positive transmission factors.
• Determine the distance a point on the circumference of a pulley travels, given the speed at which the pulley is turning.

Standards:
Principles and Standards for School Mathematics, National Council of Teachers of Mathematics (NCTM), 2000:

NCTM Algebra Standard for Grades 6-8
http://standards.nctm.org/document/chapter6/alg.htm

NCTM Algebra Standard for Grades 9-12
http://standards.nctm.org/document/chapter7/alg.htm

### Teacher Supplies

Supplies:

Teachers will need the following:

• Overhead transparency with a diagram of the fan, the crankshaft, and the alternator pulleys in an engine

Students will need the following:

• Graphing calculator
• Peg board
• Drivers and followers (at a minimum, circles with radii of 1 cm, 2 cm, 4 cm) that can be inserted into the peg board
• Rubber bands

Note: Sarah Wallick obtained the pulleys she used in the video lesson for this workshop from Glencoe McGraw Hill, publisher of the Core Plus curriculum materials.

### Teachers Activities and Assignment

Steps

Introductory Activity:

1. Display the transparency diagram on the overhead projector. Briefly discuss how the fan, crankshaft, and alternator pulleys in an engine work together.

2. With the class, discuss the following questions:

• How does the speed of the crankshaft affect the speed of the fan? How does it affect the speed of the alternator?If the idling speed of the crankshaft of a four-cylinder sports car is about 850 rpm, how far, in centimeters, would a point on the edge of the fan pulley travel in one minute? Do you think that a point on the alternator pulley would travel the same distance in one minute? Why or why not?
• Describe another situation in which the speed of one rotating object affects the speed of another object.

Learning Activities:

1. Introduce the setup of a follower and driver, and explain that students will rotate the driver pulley to determine the affect on the follower pulley. Specifically, students will determine the distance traveled by the follower when the driver pulley makes one complete revolution.

Ask students to experiment with different sizes of followers and record their observations in a two row table. They should record the number of driver rotations in the first row and the corresponding number of follower rotations in the second row.

2.Have students experiment with a driver of radius 2 cm and a follower of radius 1 cm. Check to be sure their data is close to the following: Students should graph the data in a scatterplot using a graphing calculator. Have them describe the patterns in the data, and, as a class, find algebraic models that might fit the data. Discuss various equations or representations they could use; these might include the following:

y = 2x
Next = Now + 2, Start at 0 (the value for 0 rotations of the driver)

3.Have students repeat the experiment with a driver of radius 1 cm and a follower of radius 2 cm. Their data should be close to the following: Students should look at the patterns in this second set of driver follower data. Again, they should graph the data in a scatterplot using a graphing calculator. Have them describe the patterns in the data, and, as a class, find algebraic models that might fit the data. Discuss various equations or representations they could use; these might include the following:

y = 0.5
y = 1/2 x
y = x/2
Next = Now + 0.5, Start at 0 (the value for 0 rotations of the driver)

4.Ask students to work in groups to generate a formula to determine the transmission factor for any ratio of driver-to-follower size. Reconvene the class for a discussion and select students to share and justify their answers

5. Assign student groups a set of problems related to the diagram they investigated in the introductory activity and have them prepare their solutions on poster paper. Choose several students to present solutions to the whole class. As part of this discussion, students should be able to correct, or justify, their intuitive answers to Question #2 from the introductory activity.

Culminating Activity/Assessment:

At the end of class, have students recount what they learned in their math journals. Then ask students to share their thoughts with the whole class. To summarize, create a list of things students have learned on the overhead projector.

### Related Standardized Test Questions

The questions below dealing with mathematical modeling have been selected from various state and national assessments. Although the lesson above may not fully equip students with the ability to answer all such test questions successfully, students who participate in active lessons like this one will eventually develop the conceptual understanding needed to succeed on these and other state assessment questions.

• Taken from the New York Regents Exam, Mathematics (August 2002):

In the accompanying diagram, x represents the length of a ladder that is leaning against a wall of a building, and y represents the distance from the foot of the ladder to the base of the wall. The ladder makes a 60° angle with the ground and reaches a point on the wall 17 feet above the ground. Find the number of feet in x and y. Solution: The triangle formed by the ladder, the wall, and the ground is a 30-60-90 triangle. Consequently, feet, and feet. Another solution method would be for students to use right-triangle trigonometry.

• The graph below shows the height of Cindy’s model rocket during the course of its flight. Which of these equations can be used to find the height of the rocket at any time during its flight?

A. y = 9x
B. y = x2 – 81
C. y = x2 + 9x
D. y = 9x – x2 (correct answer)

• Taken from the Virginia End of Course Exam, Algebra I (Spring 2001):

Mary published her first book. She was given \$10,000.00 and an additional \$0.10 for each copy of the book that sold. Her earnings, d, in dollars, from the publication of her book are given by:

d = 10,000 + 0.1n

where n is the number of copies sold. During the first year Mary earned \$35,000.00 from the publication and sale of her book. How many copies of her book sold in the first year?

A. 25,000
B. 35,000
D. 350,000

### Materials

The Power of the Circle” (PDF)
From Contemporary Mathematics in Context: A Unified Approach, Course 2, Part B, copyright (c) 2003 by Glencoe/McGraw-Hill. Used with permission.

### Student Work: Transmission Factors Assignment Teacher Commentary:

I’m very pleased with the work. The student has demonstrated her ability to analyze the problem and relate the data she collected in the table to a mathematical model using both an explicit equation and a recursive equation; from these she was able to develop a graph. Finally she developed a general model of transmission factor.

My only concern with the work is just below the line “Divide the driver by the follower”. She represents the relationship as a fraction, then equates it to slope. This is incorrect, since slope = delta y/delta x. The transmission factor = x/y.