Saturday, February 27, 2010

Cut A Hole About So Big..

Can you imagine how frustrating it would be to try and take measurements for a transition if you could not read a ruler? How many times have you seen students struggle to take measurements or accurately describe the size of a part or piece of equipment? For a mechanic, the inability to accurately describe what you need is a major barrier to performing your job. Make sure your students know how to intelligently discuss basic dimensions and measurements they will encounter in the field. You cannot assume that students are familiar with common dimensional concepts like area and volume. Many literally cannot read a ruler, and most do not understand the difference between the inch-pound measurement system and the SI measurement system. Although most schools use some type of entrance exam, they usually are not covering mundane practical application. I have had students who could solve algebra problems but could not read a ruler.

Find out what they know by asking them to measure something for you. Do not make it seem like a test, which will make them nervous. Just ask “John, could you do me a favor and please measure the diameter of that pipe for me?” One of the best ways for them to improve is simply to practice. First, they may need some coaching so they are not floundering. Many people have never noticed that the divisions on most tapes and rulers are arranged by height according to the units with the line for half inches being higher than quarters, which is higher than eighths, which is higher than sixteenths. Also, point out that each unit is half of the next larger unit. Although it seems obvious, many people have never made the connection that rulers just continually break things into two. I have found that students who struggle the most have never really learned to deal with fractions. Write down a scrambled list of fractional sizes less than 1 inch and ask them to order them from smallest to largest. If they can’t do it, they don’t understand fractions. A way for them to start using the ruler and learn the fractional relationships is to count the sixteenths and start out by using measurements like 12/16. Then teach them to reduce the fraction to a proper fraction. Eventually they will start to see the relationship of the different fractional values.

Many of these same students have no trouble learning to use a meter stick because it is base 10 and there are no fractions. They can easily read 45 centimeters and “three little marks” to 45.3 centimeters or 453 millimeters. When teaching SI (metric) units I purposefully try to avoid conversions from inch-pound units to SI units. The factors are invariably strange and confuse students. If your first exposure to a centimeter is that there are 2.53 of them in an inch, you wonder what demented mind came up with a system like that. I believe that the best way to teach SI measurement is simply to have the students use SI measurements. They will get a mental concept of the units by using them. Most people know what a liter is because they buy drinks in 2 liter bottles. If you asked them to tell you how many ounces equaled 2 liters they could not tell you.

There is a ruler reading game on the web that is very helpful at

There are many resources available to students on the web for students that need to improve their ability to deal with fractions. Below are a few links that can help.

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