What Is A Function



Activity 4.9 Logarithmic Scales

Overview:

The activity asks students to plot the size of several animals on a number line and then plot the log of their sizes and describe which is a better representation of the data and why it is better.

Estimated Time Required: The activity should take approximately 15 minutes.

Technology: Scientific Calculator

Prerequisite Concepts:

• Linear scale

• Logarithmic scale

• Definition of the common logarithmic function

Discussion:

The students should see that linear scales are not adequate for displaying data sets that involve a large range of values. Explain that a linear scale requires that the tick marks represent equal increments. Consider a few different linear scales and demonstrate how it is impossible to display the entire data set.

Have students label the tick marks of the second number line as powers of ten: [pic]. Then ask the students to plot the data on the new scale. To plot a point requires the students to write each size as a power of ten. For example, to write 200 as a power of ten requires the solution to the equation [pic], which is t ( 2.30103. The size of a cat would be plotted at approximately 2.3 on the log scale.

Remind students of the definition of the common logarithm function and relate the definition to the plots that they have constructed. It is very important that students understand the exact meaning of the expression “log x.” Require students to know the following definition:

log x is the exponent to which 10 must be raised to produce x.

Use some of the computations they made to illustrate the definition.

Even though all calculators have a log button, the students should understand what the output value means and they should be able to estimate logarithm values without a calculator. For example, log 0.45 is between –1 and 0 because [pic] = 0.1 and [pic] = 1.

Activity 4.9 Logarithmic Scales

Animals and their respective sizes are given in the table below.

|Animal |Size (cm) |

|Virus |0.0000005 |

|Bacterium |0.0002 |

|Human Cell |0.002 |

|Ant |0.8 |

|Hummingbird |12 |

|Domestic Cat |60 |

|Wolf (with tail) |200 |

|Thresher Shark |600 |

|Giant Squid (with tentacles) |2200 |

|Sequoia |7500 |

1. Plot the sizes for each animal on a linear scale. Number the increments on your number line and label each point.

[pic]

2. Now, find the log of each animal’s size.

|Animal |Size (cm) |Log of Size |

|Virus |0.0000005 | |

|Bacterium |0.0002 | |

|Human Cell |0.002 | |

|Ant |0.8 | |

|Hummingbird |12 | |

|Domestic Cat |60 | |

|Wolf (with tail) |200 | |

|Thresher Shark |600 | |

|Giant Squid (with tentacles) |2200 | |

|Sequoia |7500 | |

Plot the log of size values on a logarithmic scale. Number the increments on your number line and label each point.

[pic]

Which scale (linear or logarithmic) was better to use for the given data? Why

Complete the sentence:

When plotting data, logarithmic scales are better than linear scales when the data is

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