# Fundamentals of Measurement Homework

## Session 2, Homework

Problem H1

Imagine that you are playing the Between game with a partner. Player A picks a decimal number between 5 and 6 — say, 5.7 — crosses out the number 5, and writes down 5.7 in its place. Player B picks a number between 5.7 and 6, such as 5.9, crosses out the number 6, and writes down 5.9. Now Player A must pick a number between 5.7 and 5.9 — i.e., 5.8 — and replace Player A’s previous number (5.7) with it. Play continues for a total of 10 rounds (the above describes three rounds).

Imagine that the Between game was a measuring task where you were trying to become more accurate with each measurement. What does this game tell you about the nature of measurement?

Problem H2

Suppose that a scale of 1 in.:20 ft. was used for building a model train. What does this mean? If a railcar is 40 ft. long, how long is the scale model?

Problem H3

A science class wants to create insects that are larger than life. They found that a queen ant is 0.5 in. long. The large model they plan to create is 5 ft. long. What is the scale factor for the ant model?

Problem H4

Here is your car’s gas gauge: 1.  Suppose your full gas tank holds 16 gallons. Put an arrow on the gas gauge to show how much gas you would have left in your tank if you filled it up and then took a drive that used 6 gallons: 2. Your gas tank said “Empty,” but you were low on cash. You used your last \$4 to buy gas, paying \$1.139/gallon. Suppose a full tank holds 14 gallons. Put an arrow on the gas gauge to show how much gas you had in your tank after your purchase: 3. You filled up your tank this morning, then took a drive in the country to enjoy the fall colors. Your odometer said that you had gone 340 miles, and you have been averaging about 31 miles/gallon. If your gas gauge looked like this when you got home, how much does your gas tank hold when it is full? Problem H5

Suppose that you had the following 10 measurements (in centimeters) of the same object:

 31.9 32 31.9 32.1 32 32.2 32.4 32.3 32.5 32.4

1. With these data, what would you give as the best approximation? Explain why you give that approximation. What would be the precision unit for your best estimate? Think about finding the mean or an average data value of the set.

Suppose that you made five measurements in addition to the 10 listed above:

 32.1 32.2 32.3 32.3 32.4
2. What is your best approximation now?
Suppose that you made a total of 20 measurements — the 15 above and the following five:

 32 32.9 32.4 32.2 32.1
3. What is your best approximation now? Did it change?
4. In general, what effect on a best approximation do you expect if the results of more and more measurements are reported?

Problem H6

Take a piece of paper and measure its length and width. What level of precision will you use to measure? What is the accuracy of your measure in terms of the relative error? Remember, the level of precision depends on the measuring instrument and its unit of measure. Accuracy depends on relative error. You can record the accuracy as a percent by subtracting the relative error from 1 and writing the resulting decimal as a percent.

### Solutions

Problem H1

The fact that this game does not reach an “ending” (with no moves available) shows that there can always be a more accurate measurement, and therefore the act of measuring can never be exact.

Problem H2

This means that all lengths in the model are 1 in. long for every 20 ft. in the original model. If the original railcar is 40 ft. long, we can set up the following proportion to calculate the model’s length: Solving the equation for x would give us x = 2. So the model’s length will be 2 in.

Problem H3

The scale factor is 5 ft.:1/2 in. For easier calculations, we can convert the ratio into inches. Since 1 ft. = 12 in., the scale factor would be 60:1/2, or 120:1.

Problem H4

 a. One way to do this is to partition the gas gauge into 16 equal parts by dividing it in half four times (2 • 2 • 2 • 2 = 16). You can then count 6 gallons. Another way is to note that 6/16 = 3/8, so partitioning the gauge into eighths is sufficient. b. One way to solve this problem would be as follows: Filling up the entire tank would cost \$1.139 • 14 = \$15.94. Four dollars is one-fourth of \$16, so it is just a little more than one-fourth of \$15.94. The arrow should be just slightly above one-fourth full. c. First you need to find out how much gas you used for 340 miles: 340 31 = 10.97 So approximately 11 gallons were used. Next, look at the needle position: It is approximately one-third of the way from “Empty” to the first quarter mark. Further subdividing the gauge into 12ths, we see that the needle is about 1/12 of the way from “Empty.” Since about 11 gallons were used, and 11/12 of the tank is empty, the tank therefore holds about 12 gallons when full. Problem H5

 a. The best approximation might be about 32.2 cm, which is in the center of the data set. The average (mean) of the 10 measurements is 32.17 cm. The precision unit appears to be tenth (or 1 mm), as all 10 measurements are recorded to the nearest 0.1 cm. b. The mean of the 15 measurements is 32.2 cm. Again, the precision unit might be 0.1 cm. Our approximation, however, is better now since we have more measurements. c. The mean is now 32.23 cm. Even though the measurement errors are pretty high, this gives us even more confidence that the actual measure is close to 32.2 cm. d. With more measurements we will get an increasingly better approximation.

Problem H6

The level of precision depends entirely on the measuring instrument. If we use a measuring stick with 1-in. precision, the maximum error is 0.5 in.

The accuracy can be obtained by subtracting the relative error from 1 and writing the resulting decimal as a percent. For the longer side of the paper, the relative error is 0.5/11, or about 0.045 (so, the accuracy is 1 – 0.045 = 0.95, or 95%). The relative error for the shorter side is 0.5/8.5, or about 0.059 (so the accuracy is 1 – 0.059 = 0.94, or about 94%), which is less accurate. A measuring stick with more partitions will give more precise and more accurate measurements.