2.1: Measuring Length
Introduction
The Tomato Plants
Tania and her brother Alex have decided to plant a vegetable garden. They are interested in eating more vegetables, they have a big yard to work with, and they want to have some of their vegetables this summer to make extra money.
Tania is in charge of selecting the seeds and beginning some of the seedlings inside until it is warm enough to move them outside. Alex is working on the outside garden design.
Tania has decided to begin with tomatoes. It is early spring, so she knows that it is probably the best time to begin. She has gathered her supplies and a pack of seeds.
Tania begins reading the package and learns that there are all kinds of measurement issues when planting seeds.
The package says that she should plant each seed \begin{align*}\frac{1}{8}\end{align*}" or 3 mm deep.
It also warns that if the plants get too much sunlight that they will germinate to have \begin{align*}3''\end{align*} long stems. This is undesirable because the stems are too long.
Tania wants her tomatoes to grow the correct way.
What tool should she use to measure with?
What unit makes the most sense? The package says inches and millimeters.
In this lesson, you will learn all about helping Tania with her tomatoes.
Pay close attention to the measurement details here and you will know whether Tania is on the right track.
What You Will Learn
In this lesson, you will learn to:
- Measure length in customary units.
- Measure length using metric units.
- Choose appropriate tools given measurement situations
- Choose appropriate units for given measurement situations.
Teaching Time
I. Measuring Length in Customary Units
What is measurement?
Measurement is a concept that appears all the time in everyday life. How far apart are two houses? How long is a basketball court? How far is the Earth from the sun? Sometimes we need to measure a long distance and sometimes we need to measure very short distances. You have probably measured many things before in your life.
If we measure length, we measure the distance between two points, two things or two places. For the tomato plant question, we want to measure how long something is. To measure length, we need to use a unit of measure.
Let’s begin by learning about the Customary Units of measurement for measuring length.
The most common Customary Units of measurement are the inch, the foot, the yard and the mile.
The inch is the smallest of these units of measurement.
There are 12 inches in 1 foot.
There are 3 feet in 1 yard.
There are 5,280 feet in 1 mile.
Whew! That is a lot of measuring. Let’s go back to the inch and work with that one first.
Inches
One inch is roughly the length of your thumb from the tip to the knuckle. The ruler below is shows inch long segments (not shown to actual scale).
We can measure small things in inches. That is what makes the most sense. Here is a picture of a crayon. Let’s look at how long the crayon is in inches.
We can also divide up the inch. An inch can be divided into smaller units. We can divide the inch into quarters. Look at this ruler. We can see \begin{align*}\frac{1}{4}'', \frac{1}{2}'', \frac{3}{4}''\end{align*}.
Beyond that, we can measure things as small as eighths. This means that each inch can be divided into 8 units. Two times this smallest unit is one fourth of an inch.
Let’s look at what one fourth of an inch looks like on a ruler.
We use inches and parts of inches to measure small items.
Feet
The next unit we use is the foot. To abbreviate the foot we write \begin{align*}ft\end{align*} (for example, 3 ft). One foot is roughly the length from your elbow to the end of your fingers.
We can use a ruler to measure feet, because a ruler is exactly one foot long.
As you can see, one foot is much longer than one inch. We therefore use feet to measure bigger objects, such as the height of a door or the length of a car. We can also use feet to measure the distance between things. When two people stand apart, it would take a lot of inches to measure the distance between them. In this case, we can use feet.
Yards
A unit of measurement that you will sometimes hear about is yards. There are three feet in one yard. You can think about yards as being a measurement shortcut. Let’s look at an example.
Example
The rope was 2 yards long.
How long was the rope in feet?
Well, you can think about this mathematically.
If the rope was 2 yards long and there are 3 feet in every yard then we can multiply to figure out the number of feet that the rope is.
3 \begin{align*}\times\end{align*} 2 \begin{align*}=\end{align*} 6
The rope is 6 feet long.
It makes sense to use inches, feet and yards when measuring short distances or the length of objects or people. We use these customary units of measurement all the time in our everyday life.
What happens when we want to measure long distances-like the distance between two houses or two cities?
It would be very complicated to use feet or yards to figure this out.
In a case like this, we use our largest customary unit of length-the mile.
Miles
There are 5,280 feet in one mile. The best thing for you to remember about miles right now is that miles are used to measure very long distances.
Later in this lesson, you will be using miles to measure the distances between places.
Here are few things for you to measure in inches. Find examples of these things and measure them.
- Pencil
- Sneaker
- Desk Height
Take a minute to check your work with a peer.
II. Measuring Length Using Metric Units
In science classes, and anywhere outside of the United States, we measure length with the metric system. The most common units that we use to measure length in this system are the millimeter, centimeter, meter, and kilometer. This section will give you an overview of each measurement unit. Let’s take a look at each.
Millimeter
The millimeter is the smallest commonly used unit in the metric system. When we measure something in millimeters, we use \begin{align*}mm\end{align*} as an abbreviation for millimeter.
A millimeter would be used to measure something that is very small, like a seed.
Centimeter
The centimeter is the next smallest unit of measurement. To abbreviate centimeters we write \begin{align*}cm\end{align*} (for example, 3 cm). Centimeters are smaller than inches. There are 10 millimeters in a centimeter. This ruler shows centimeters.
We can use a ruler to measure centimeters and millimeters. On many rulers, we can see both units of measurement and the metric units of measurement.
You can see inches, centimeters and millimeters on this ruler.
What about when we have to measure something that is longer than a ruler?
When we are measuring something that is longer it doesn’t make sense to use centimeters or millimeters. We could use them, but it would take a very long time to count all of those centimeters or millimeters. Instead, we can use two larger units of measurement. We can use the meter and the kilometer.
Meters
A meter is longer than a foot and is about the same length as a yard. To abbreviate the meter we write \begin{align*}m\end{align*} (for example, 8 m).
One meter is roughly the length from your finger tips on one hand to the fingertips on your other hand if you stretch your arms out to your sides.
Go ahead and try this right now with a peer.
As you can see, one meter is much, much longer than one centimeter. It takes 100 centimeters to equal one meter.
We use meters to measure bigger objects or longer distances, such as the depth of a pool or length of a hallway and use a meter stick to measure meters.
This is a bit complicated, however, when an object or distance is several meters long.
We have to make a mark on the object being measured at the end of the meter stick, then move the meter stick down and make another mark to show the next meter.
It is easier to use a tape measure.
Tape measures often show customary units (feet and inches) down one side and metric units (centimeters and meters) down the other.
What about when we want to measure much longer distances and it doesn’t make sense to use meters?
That is when we use kilometers.
Kilometers
Kilometers are very long. To abbreviate the word kilometer we write \begin{align*}km\end{align*} (for example, 12 km).
Like miles, we use kilometers to measure long distances, such as the distance from your house to the store or from one town to another.
Kilometers are only a little more than 1/2 as long as a mile. There are 1,000 meters in a kilometer!
Here are a few small items for you to practice measuring using millimeters and centimeters. We will be working with meters and kilometers a little later.
III. Choosing the Appropriate Tool Given Measurement Situations
Wow, that last section had a lot of information in it. Now that you have an overview of units of measurement, we can look at applying this information in real life situations.
The first thing that you have to look at when you are trying to measure something is the tool that you are using.
What kinds of tools can we use to measure length?
We have already talked about a couple of different tools. Let’s look at those and some that we haven’t talked about yet.
- Rulers
- Tape measures
- Yard sticks
- Meter sticks
Rulers
Rulers are used all the time in mathematics. We can use a ruler to measure things that are small. Most rulers show both customary units of measurement like inches and metric units of measurement, such as the millimeter and the centimeter.
When you measure something that is small, a ruler is often the best tool to use.
You can measure the small item in customary units or metric units or both.
Let’s look at an example.
Example
We can see that this barrette is about ______ inches long.
The barrette can also be measured in centimeters. It is about __________ centimeters long.
If an item that is being measured fits on a piece of paper, a ruler is probably the best tool to use.
Tape Measure
If we were going to measure the length of a table, we could use a ruler, but it is probably not the best tool to use.
Think about it. A table is probably much longer than a ruler. We could line up ruler after ruler after ruler, but this would be a bit time consuming.
There is an easier way. We can use a tape measure.
Tape measures are used to measure many of the distances that are too long for a ruler to measure easily.
We use tape measures to measure the distance across a room or an object that is very long.
Tape measures show us length in inches and feet. We can see exactly how long something is by comparing the length of the object with the measurement on the tape measure.
Yard Stick
What about a yard stick?
A yard stick measures things by the yard.
Since there are three feet in a yard, we can say that a yard stick could be used for things that are longer than a piece of paper, but not too long.
Some of the things that we could measure with a tape measure we could also measure with a yard stick.
Think about the table in the last example.
We could also use a yard stick to measure it.
Sometimes, you have to use common sense. If the table is really long, longer than the yard stick, then you would switch to the tape measure.
Meter Stick
A meter stick measures one meter.
We can use meter sticks to measure objects that are larger than a piece of paper.
Remember that you can use centimeters and millimeters if the object is smaller than a piece of paper. Those items are easily measured with a ruler.
A meter stick is actually a little over 3 feet long, because a meter is approximately 3.2 feet.
A meter stick compares to a yard stick.
For instances where we would use a yard stick for customary units, we can use a meter stick for metric units.
If we were measuring a table length in metric units, a meter stick would probably be our best choice.
Here are few examples for you to try on your own.
Which tool would be the best tool for you to use if you were measuring each of the following items?
- A toothpick
- The length of a room
- The height of a standing lamp
Take a few minutes to check your work with a peer.
IV. Choosing Appropriate Units for Given Measurement Situations
We just finished looking at the different tools that we can use to measure in different situations.
Now, we are going to look at using the best unit of measurement for different situations and items.
When do we use inches, centimeters or millimeters?
It is best to think about using inches, centimeters or millimeters when we have small items to measure.
Items smaller than a piece of paper are often best measured in inches, centimeters or millimeters.
Example
Which unit would you use to measure the length of a bug that needs to be seen with a magnifying glass?
Let’s think about this. If the bug is so tiny that it needs to be seen with a magnifying glass then it is probably smaller than inches and centimeters.
A millimeter is the best unit to measure this bug.
When do we use meters, yards or feet?
Part of this question depends on whether you want to measure things in customary units or metric units.
First, let’s think about feet.
When we have items or objects that are bigger than a piece of paper, we can use feet to measure them.
We often measure the length of large objects like tables or walls or the length of a room in feet.
What about yards and meters?
Since a yard is equal to 3 feet, and a meter is equal to a little more than 3 feet, we can sometimes more easily use yards or meters instead of feet.
Again, you have to decide if you are using customary units or metric units.
A room that is 6 feet long could also be measured in yards or meters.
For customary units, we use yards, 6 feet = 2 yards.
For metric units, we use meters, 6 feet is a little less than 2 meters.
Example
What unit of measure would you use to measure the height of a fence?
Think about this one. A fence is definitely larger than a piece of paper.
If we wanted to measure it in metrics, we would use meters.
If we were measuring it in customary units, we would probably use feet.
When do we use miles or kilometers?
Miles and kilometers are used to measure longer distances.
Think about how difficult it would be to use meters to measure the distance from your house to your school.
It would be so many meters that we wouldn’t really be able to get a good sense of how far it is.
That is the reason why we use a larger unit of measure.
It gives us a better sense of how far something is.
Miles and Kilometers are used to measure distances between cities, houses, places on a map.
Here are a few measurement questions for you to try on your own.
Choose the best unit of measurement for each item. Write both the customary unit and the metric unit.
- A road race
- The length of a back yard
- The length of a large screen television set
Real Life Example Completed
The Tomato Plants
Now that we have learned all about measurement, we are ready to help Tania with her tomato plants.
Let’s look at the problem once again.
Tania and her brother Alex have decided to plant a vegetable garden. They are interested in eating more vegetables, they have a big yard to work with, and they want to have some of their vegetables this summer to make extra money.
Tania is in charge of selecting the seeds and beginning some of the seedlings inside until it is warm enough to move them outside. Alex is working on the outside garden design.
Tania has decided to begin with tomatoes. It is early spring, so she knows that it is probably the best time to begin. She has gathered her supplies and a pack of seeds.
Tania begins reading the package and learns that there are all kinds of measurement issues when planting seeds.
The package says that she should plant each seed \begin{align*}\frac{1}{8}''\end{align*} or 3 mm deep.
It also warns that if the plants get too much sunlight that they will germinate to have 3” long stems. This is undesirable because the stems are too long.
Tania wants her tomatoes to grow the correct way.
What tool should she use to measure with?
What unit makes the most sense? The package says inches and millimeters.
First, we underline all of the important information.
Now that we have done that, the next thing that Tania needs to do is to choose a tool to measure with.
The seed is very small. Tania sees that the package talks about using inches and millimeters.
The seed is smaller than a piece of paper, so Tania is going to need a ruler to measure with.
Tania takes out her small planter and fills each container with planting soil. She takes her ruler and begins to measure about how deep each seed should go in the pot.
Should Tania use inches or millimeters?
This is Tania’s personal choice. But since the plants go \begin{align*}\frac{1}{8}''\end{align*} deep, it might be easier to use millimeters.
Tania takes a ruler and measures 3 mm on the plant pot. Then she plants the seed.
You can look at a ruler and find 3 mm on it. This will help you to see the length of Tania’s measurement.
Tania’s next concern is the length of the stem after germination. Tania does not want the stems to be long and leggy.
Tania decides to use inches to measure the stems as her plants grow.
This way she can be sure that they are the correct size when replanted.
Tania has started her tomato plants.
Alex has the next task. He is working to design the outside garden plot.
He will need the measurement skills from this lesson plus some new ones in the next lesson.
Vocabulary
Here are the vocabulary words that are in this lesson. You can find them throughout the lesson in italics.
- Measurement
- using different units to figure out the weight, height, length or size of different things.
- Length
- how long an item is
- Customary units of length
- units of measurement such as inches, feet, yards and miles
- Metric units of length
- units of measurement such as millimeter, centimeter, meter and kilometer.
- Inches
- the smallest customary units of measurement, measured best by a ruler
- Foot
- a customary unit of measurement, there are 12 inches in 1 foot
- Yard
- a customary unit of measurement, there are 3 feet in 1 yard
- Mile
- a customary unit for measuring distances, there are 5280 feet in 1 mile
- Millimeter
- the smallest common metric unit of length
- Centimeter
- a small metric unit of length, best measured by a ruler
- Meters
- a unit compared with a foot or yard. 1 meter = a little more than 3 feet
- Kilometer
- a metric unit for measuring distances
Technology Integration
Khan Academy Adding Different Units of Length
Other Videos:
http://www.linkslearning.org/Kids/1_Math/2_Illustrated_Lessons/2_Estimation_of_Length/index.html – This website has a video which shows you all kinds of things about estimating length. It is a very fun, visual website.
Time to Practice
Directions: Write the appropriate customary unit of measurement for each item.
1. The height of a sunflower
2. The depth to plant a seed in the soil
3. The height of a tree
4. The area of a garden plot
5. The distance from a garden to the local farm store
6. The length of a carrot
7. A stretch of fencing
8. The length of a hoe
9. The distance between two seedlings planted in the ground
10. The height of a corn stalk
Directions: Choose the appropriate unit of length using metric units for each item listed below.
11. A grub collected from the garden
12. The width of a garden row
13. The length of a garden row
14. The size of a small seedling
15. The distance that a tractor can travel on a large farm per day
Directions: Choose the appropriate tool to measure each item in metrics and customary units.
16. The height of a light switch
17. The width of a refrigerator
18. The measurements of a placemat
19. The length of a pencil
20. The width of a chapter book
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