Rewrite: A Parent’s Survival Guide To Physics by Me’ira Pitkapaasi page 1

Topic One: Measuring Length Part 1: Inches, feet, yards, centimeters, meters

Measurement in Physics is not quite as simple as taking out your measuring tape. This tool will come in handy occasionally, such as when you’re determining lengths that are large enough to see and short enough to personally reach. On our measuring tape, that would be 1/8th of an inch to a yard; or a millimeter to a meter. In the greater scientific community, we use the metric system, so in order to understand what your student is learning, we’re going to figure out how this system works! An inch is typically the smallest unit of length used by the average parent in the U.S.A., or a fraction thereof. We were always taught that an inch is about two and a half centimeters, which was correct!

1 in (inch) = 2 ½ cm (centimeters)

or, more accurately,

1 in = 2.54 cm

So how do we compute length from there? We use a formula that you can understand if you can compute multiplication and division problems with your calculator, and you use the above equivalencies.

_____ centimeters = (2.54 centimeters) x (however many inches you just measured) ÷ (1 inch)

Or

______ = 2.5 x (measured number) ÷ 1

Or

If you’re measuring length of a standard piece of copy paper, you’re going to find that it’s 11 inches long.

________ = 2.54 x 11 ÷ 1 ________ = 27.94 cm

The length of your 11 in paper is also 27.94 cm. Pretty cool, eh?

What if we’re measuring feet though? There are 30.48 centimeters in a foot. The formula changing feet to cm is very similar to the formula above:

______ centimeters = (30.48 centimeters) x (however many feet you just measured) ÷ (1 foot)

______ = 30.5 x (measured number of feet) ÷ 1

If you measured how tall your living room is, you might have discovered it was 7 feet tall. Let’s put it into our equation:

______ = 30.48 x (7) ÷ 1 _______= 213.36 centimeters.

The height of your living room is 213.36 cm! 213.36 cm is an awkward measurement. It’s quite inconvenient to measure the height of your living room by a couple of hundred units. We use this equivalency:

100 centimeters = 1 meter

With this equivalency we create a new formula:

_______ m (meters) = (how ever many cm you are using) ÷ 100 cm x 1 m

Using our above measurement of the height of your living room,

_______ m = (213.36 cm) ÷ 100 cm x 1 m

_______ m = 2.1336 ÷ 1 ________ = 2.1336 m

A couple of meters tall is a lot easier to measure than a couple of hundred centimeters.

New formulas for longer measurements! How about a football field?

Football fields are a bit long for your tape measure, but we know they’re 100 yards long. Knowing that 214 centimeters as the height of your living room is a bit too high of a number to use conveniently certainly tells us that centimeters is not an appropriate measurement for a football field. That’s check just to make sure, knowing that 1 yd (yard) = 91.44 cm.

_______ cm = (how many yards you have measured) ) x 91.44 cm ÷ 1 yd (yards)

_______ cm = 100 yards x 91.44 cm ÷ 1 yd

_______ cm = 100 x 91.44 ÷ 1 ______ = 9,144 cm

This certainly shows that measuring a football field in centimeters isn’t the greatest method of measurement. Who wants to count out nine thousand centimeters? Let’s move on to the meter. Just as we were often told an inch is around 2 ½ centimeters, we were often told that a meter was “a little more than a yard.” Once again, fairly accurate. One meter = 1.09 yards. So how do we find the length of a football field?

______ meters = (1 meter) x (number of yards measured) ÷ (1.09 yards)

______ meters = 1 x (number of yards measured) ÷ 1.09

______ meters = 1 x 100 ÷ 1.09 _______ = 109 meters

Were you wondering where all of these formulas came from? Equivalencies and Cross Multiplication are the two methods used to create these and many other Physics formulas. Topic Two will cover these two methods.

Next passage: Topic 2: Equivalencies and Cross Multiplication