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Pre-Algebra View details
7. Convert Measurement Units
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Chapter 3
7. 

Convert Measurement Units

Understanding measurement conversion is crucial in various fields, from engineering to daily life. The lesson emphasizes the importance of being proficient in converting units, especially within the metric system. It also delves into the concept of unit ratios, illustrating their practical applications like determining speed or converting dimensions of objects. This knowledge serves as a comprehensive guide for those involved in activities that require a nuanced understanding of unit conversions, making it easier to meet specific criteria or standards.

Student Learning Objectives:
  • Use unit ratios to compare values
  • Use conversion factors
  • Compare customary and metric units
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14 Theory slides
10 Exercises - Grade E - A
Each lesson is meant to take 1-2 classroom sessions
Convert Measurement Units
Slide of 14
Measuring is the process of using numbers and units to describe things like length, weight, or space. We use a variety of units to measure things. For example, distance can be measured in kilometers or miles. This lesson explains how to convert from one unit to another.

Catch-Up and Review

Here are a few recommended readings before getting started with this lesson.

Challenge

Different Measures: Comparing Average Speeds

Ignacio and Emily are competing in an international remote control car race.

Ignacio car traveled 0.4 miles in 2 hours. Emily's car traveled at an average speed of 0.4 kilometers per hour. Whose car drove at a higher average speed?

Discussion

Unit Ratio

A unit ratio is a ratio with a denominator of 1 unit.

A ratio is a comparison of two quantities with similar units of measure, but unit ratios can compare quantities with different units of measure. Unit ratios can be simplified to 1.

When writing unit ratios using different units of measure, the denominator of the unit ratio is still 1 unit. Note that different ratios can be written using the same relationships.

Fact 1 foot is equal to 12 inches. 1 hour is equal to 60 minutes.
Ratio 1ft/12in. 1h/60min

The denominators of these ratios are different than 1 unit. Dividing both the numerator and denominator of each ratio by the number in the denominator will give the appropriate unit ratio.

These resulting unit ratios mean that 1 inch is approximately 0.083 feet and 1 minute is approximately 0.017 hours. Unit ratios are useful when converting measurement units.
Pop Quiz

Practice Rewriting Ratios

Write the given ratio in the indicated form. Do not round any decimal numbers.

Randomly generated ratio
Example

Converting the Map's Distance to the Actual Distance

Ignacio found a map of his school's campus.

Road map
The sign in the map's bottom right corner means that 1 inch on the map corresponds to 0.075 miles of actual distance on the school's grounds. How many miles does 1.4 inches on the map represent in reality? Round the answer to three decimal places if necessary.

Hint

Write a ratio using the fact that 1 inch on the map represents 0.075 miles in real distance.

Solution

We know that 1 inch on the map represents 0.075 miles in reality. To find the distance in reality that corresponds to 1.4 inches, we can multiply 1.4 by a ratio based on the map's scale. ccc On the Map & & Actual Distance 1in. & : & 0.075 mi Since the given distance 1.4 is in inches, the denominator of the ratio will be in inches, so the inch units cancel out. The numerator will contain the corresponding distance in miles. 0.075 mi/1in. Let's multiply 1.4 inches by this ratio to get the distance in miles.

1.4 in. * 0.075 mi/1in.
1.4 in. * 0.075 mi/1in.

Cross out common units

1.4 in.* 0.075 mi/1 in.

Cancel out common units

1.4 * 0.075 mi/1
0.105 mi/1
0.105 mi

This means that 1.4 inches on the map represents 0.105 miles in real life.

Discussion

Conversion Factor

A conversion factor is a fraction where the numerator and denominator represent the same quantity with different units. Example Conversion Factor [0.5em] 60 minutes/1 hour Recall that 1 hour and 60 minutes represent the same quantity. Multiplying a quantity by a conversion factor changes the quantity to an equivalent quantity in different units.

Given Quantity Conversion Result
2 hours 2 hours * 60 minutes/1 hour 120 minutes

The process of including units of measurement as factors is called dimensional analysis. Dimensional analysis can also be used when deciding which conversion factor will produce the target units.

Why

The Reason That the Quantities Are Equivalent
The numerator and denominator of the conversion factor represent the same quantity. This means that their quotient equals 1. Then, by the Identity Property of Multiplication, the amount of the given quantity does not change when multiplied by the conversion factor. Given Amount * 1 = Given Amount When converting from one unit to another, the target unit needs to be in the numerator of the conversion factor and the given unit needs to be in the denominator. Then when the quantity is multiplied by the conversion factor, the given unit will cancel out and the target unit will remain.

Discussion

Customary System

The customary system is the measurement system most commonly used in the United States. This system of measurement contains units for length, capacity, and weight. For example, the inch is a unit of length, the ounce is a unit of weight, and the quart is a unit of volume. Examples of Units in the Customary System inch, ounce, and quart The table shows the relationship between units of each measure type in the customary system.

Customary Units
Type Unit Equivalent Unit
Length 1 foot (ft) 12 inches (in.)
1 yard (yd) 3ft
1 mile (mi) 5280ft
Weight 1 pound (lb) 16 ounces (oz)
1 ton (T) 2000 lb
Volume 1 cup (c) 8 fluid ounces (fl oz)
1 pint (pt) 2c
1 quart (qt) 2pt
1 gallon (gal) 4qt
Converting measures requires using the appropriate conversion factors.
Example

Determining the Weight of the Robotic Car

Remote control cars in a robotics competition must weigh less than 6 pounds. Tearrik's car currently weighs 84 ounces.

Boy-making-robot.jpg

a

Find the weight of the car in pounds.

b

Does the weight of this robotic car meet the requirement?

Hint

a

Write a conversion factor using the fact that 1 pound is 16 ounces.

b

Is the answer from Part A less than 6 pounds?

Solution

a

To convert the weight of the remote control car from ounces to pounds, we can use a conversion factor. Let’s first recall that 1 pound is 16 ounces.

1lb = 16oz Since our target unit is pounds, it goes in the numerator of the conversion factor. The denominator will have the same unit as the given amount, ounces, so the units cancel out. Conversion Factor Ounces → Pounds [0.8em] 1 lb/16 oz Now we can use this conversion factor to write 84 ounces in pounds.

84 oz * 1lb/16 oz
84 oz * 1lb/16 oz

Cross out common units

84 oz * 1lb/16 oz

Cancel out common units

84 * 1 lb/16
84 lb/16
84/16 lb
5.25 lb

Tearrik's car weighs 5.25 pounds.

b

We found that Tearrik's car weighs 5.25 pounds, which is less than the competition's maximum weight of 6 pounds. This means that Tearrik's car meets the weight requirement.

Discussion

Metric System

The metric system is a measurement system commonly in most countries around the world. The base units in the metric system are meters for length, liters for capacity, and kilograms for weight. Base Units in Metric System meter, liter, and kilogram In the metric system, units are related by powers of 10. Metric units are named by adding metric prefixes to the base units.

Metric Units of Length

The table shows the commonly used metric units of length.

Unit Equivalent Unit
1000 millimeters (mm) 1 meter (m)
100 centimeters (cm) 1m
10 decimeters (dm) 1m
1 dekameter (dam) 10m
1 hectometer (hm) 100m
1 kilometer (km) 1000m

Metric Units of Capacity

For measuring capacity, the metric system uses the liter as the base unit.

Unit Equivalent Unit
1000 milliliters (mL) 1 liter (L)
100 centiliters (cL) 1L
10 deciliters (dL) 1L
1 dekaliter (daL) 10L
1 hectoliter (hL) 100L
1 kiloliter (kL) 1000L

Metric Units of Weight

In the metric system, kilogram, gram, and milligram are some commonly used units for measuring weight.

Unit Equivalent Unit
1000 milligrams (mg) 1 gram (g)
100 centigrams (cg) 1g
10 decigrams (dg) 1g
1 dekagram (dag) 10g
1 hectogram (hg) 100g
1 kilogram (kg) 1000g
Notice that each relationship in the tables can be written as a ratio. These ratios can be considered conversion factors.
Example

The Length of Emily's Car

Remote control cars in a robotics competition must be no longer than 40 centimeters.

Two-girls-making-robot.jpg

a

Emily's car is 0.35 meters long. Write this length in centimeters.

b

Does this car meet the length criteria?

Hint

a

Write a conversion factor using the fact that 1 meter is 100 centimeters.

b

Is the answer from Part A longer than 40 centimeters?

Solution

a

To convert the given length from meters to centimeters, we can use a conversion factor. Let's first recall that 1 meter is 100 centimeters.

1m = 100 cm Since the target unit is centimeters, we will put 100cm in the numerator of the conversion factor. The denominator will contain its equivalent length, 1m. 100cm/1m Now we can convert the given quantity from meters to centimeters by multiplying it by this conversion factor.

0.35 m * 100cm/1 m
0.35 m * 100cm/1 m

Cross out common units

0.35 m * 100cm/1 m

Cancel out common units

0.35 * 100 cm/1
0.35 * 100 cm
35 cm

The length of the car is 35 centimeters.

b

Emily's car is 35 centimeters long, which is less than 40 centimeters, so her car meets the competition's length requirement.

Discussion

Converting Measures Between Systems

Units can be converted between the customary system and the metric system using conversion factors. Use the applet below to explore several common conversion factors.

Conversion factors between different units of measurement
Example

Unit Conversion Practice

Fill in the table to convert between units.

Applications
Car Weight Length
Car 1 5.3 pounds A kilograms 15 inches B centimeters
Car 2 C pounds 2.5 kilograms D inches 35 centimeters

If necessary, round answers to the nearest tenth.

Hint

Remember, 1 kilogram is about 2.2 pounds and 1 inch is 2.54 centimeters.

Solution

The measurements given for Car 1 are in customary units, but they are in metric units for Car 2. We want to convert between them. Let's start with the weights.

Finding Missing Weights

The weights are in kilograms and pounds. Let's recall that 1 kilogram is about 2.2 pounds. We can create a conversion factor using this information.

Finding A

Car 1 weighs 5.25 pounds, so our conversion factor should have a numerator in kilograms and a denominator in pounds. 1kg/2.2lb Now we can multiply the weight of Car 1 by this conversion factor.

5.3 lb * 1kg/2.2 lb
5.3 lb * 1kg/2.2 lb

Cross out common units

5.3 lb * 1kg/2.2 lb

Cancel out common units

5.3 * 1 kg/2.2
5.3 kg/2.2
5.3/2.2 kg
2.409090 ... kg
≈ 2.4

Car 1 weighs about 2.4 kilograms, so the value of A is 2.4.

Finding C

Now we want to convert the weight of Car 2 from kilograms to pounds, so we can use the multiplicative inverse of the previous conversion factor. 2.2lb/1kg Let's use this factor to find the equivalent weight in pounds.

2.5 kg * 2.2lb/1 kg
2.5 kg * 2.2lb/1 kg

Cross out common units

2.5 kg * 2.2lb/1 kg

Cancel out common units

2.5 * 2.2 lb/1
2.5 * 2.2 lb
5.5 lb

Car 2 weighs 5.5 pounds, so the value of C is 5.5.

Finding Missing Lengths

This part requires converting between inches and centimeters. Let's remember that 1 inch is 2.54 centimeters.

Finding B

Car 1 is 15 inches long. To convert this length to centimeters, we can use the following conversion factor. 2.54cm/1in. Now we can multiply the given length by this conversion factor to find its equivalent length in centimeters.

15 in. * 2.54cm/1in.
15 in. * 2.54cm/1 in.

Cross out common units

15 in. * 2.54cm/1 in.

Cancel out common units

15 * 2.54 cm/1
15 * 2.54 cm
38.1 cm

Car 1 is 38.1 centimeters long, so the value of B is 38.1.

Finding D

Lastly, we convert the length of Car 2 from centimeters to inches. To do this, we multiply 35 cm by 1 in. 2.54cm.

35 cm * 1 in./2.54cm
35 cm * 1in./2.54 cm

Cross out common units

35 cm * 1in./2.54 cm

Cancel out common units

35 * 1 in./2.54
35 in./2.54
35/2.54 in.
13.779527 ... in.
≈ 13.8 in.

Car 2 is about 13.8 inches long, so D is 13.8. Let's fill in the table with our findings.

Applications
Car Weight Length
Car 1 5.3 pounds 2.4 kilograms 15 inches 38.1 centimeters
Car 2 5.5 pounds 2.5 kilograms 13.8 inches 35 centimeters
Example

Speeds of Robotic Cars

Two remote control cars race around a 80-foot track.

a

Car A completed one lap in 5 minutes. Find the speed of the car in inches per second.

b

Car B completed 3 laps in 16 minutes. Find the speed of the vehicle in centimeters per second.

c

Which car will likely win?

Hint

a

The speed of an object is the distance traveled divided by the time elapsed. Remember that 1 foot is 12 inches.

b

Keep in mind that 1 foot is 30.48 centimeters.

c

Determine how many feet each car travels in one minute.

Solution

a

The speed of an object is calculated by dividing the distance traveled by the amount of time spent traveling. Since one lap is 80 feet and Car A completes a lap in 5 minutes, we can write its speed as follows.

r = 80 ft/5min Notice that this unit is feet per minute. To convert this to inches per second, we need to combine two conversion factors.

Equivalent Quantities Conversion Factor
1 ft = 12 in. 12 in./1ft
1 min = 60 sec 1 min/60sec

Let's multiply the speed by the conversion factors.

80ft/5 min * 12 in./1ft * 1 min/60sec
80ft * 12 in. * 1 min/5 min * 1ft * 60sec

Cross out common units

80 ft * 12 in. * 1 min/5 min * 1 ft * 60sec

Cancel out common units

80 * 12 in. * 1/5 * 1 * 60 sec
80 * 12 in./5 * 60 sec
960 in./300 sec
3.2 in./sec

The speed of the car is 3.2 inches per second.

b

Car B completed 3 laps in 16 minutes. Since one lap is 80 feet, 3 laps is equal to 240 feet. We can use this information to find the speed of the car in feet per minute.

r = 240 ft/16min To convert it to centimeters per second, we need two conversion factors.

Equivalent Quantities Conversion Factor
1 ft = 30.48 cm 30.48 cm/1ft
1 min = 60 sec 1 min/60sec

Again, we multiply the speed by the conversion factors.

240ft/16 min * 30.48 cm/1ft * 1 min/60sec
240ft * 30.48 cm * 1 min/16 min * 1ft * 60sec

Cross out common units

240 ft * 30.48 cm * 1 min/16 min * 1 ft * 60sec

Cancel out common units

240 * 30.48 cm * 1/16 * 1 * 60 sec
240 * 30.48 cm/16 * 60 sec
7315.2 cm/960 sec
7.62 cm/sec

Car B travels at a speed of 7.62 centimeters per second.

c

To find which car is more likely to win the race, we can compare their speeds. Let's use the speeds in feet per minute that we found in the previous parts.

Car A Car B
Speed = Distance/Time 80 ft/5min 240 ft/16min
Simplify 16ft/min 15ft/min

Car A is faster, so it is more likely to win the race.

Alternative Solution

Alternative Solution of Part C
We can solve Part C in a different way. We can use the answers from Parts A and B to compare the speeds of the cars, but we need to convert between inches and centimeters.

Car A Car B
Speed 3.2in./sec 7.62cm/sec

To compare these quantities, let's use the fact that 1 inch is 2.54 centimeters. We will multiply the speed of Car A by 2.54 cm1 in. to convert it to centimeters per second. 3.2in./1 sec * 2.54 cm/1 in. = 8.128cm/1 sec Car A travels at 8.128 centimeters per second, which is greater than Car B's speed of 7.62 centimeters per second.

Closure

Different Measures: Comparing Average Speeds

Earlier we were given the distance traveled by two different remote control cars.

Travels
Ignacio's Car 0.4 miles in 2 hours
Emily's Car 0.4 kilometers per hour

Since speed is distance traveled divided by time, the speed of Ignacio's car is 0.4 miles divided by 2 hours. Ignacio's Car [0.7em] 0.4 miles/2 hours = 0.2miles/1 hour Ignacio's car travels at 0.2 miles per hour. Emily's car's speed is given in kilometers per hour, however. To compare two speeds with different units, one of them must be rewritten in terms of the other. Let's use the conversion factor between kilometers and miles. 1mile is about1.6kilometers. ⇓ Conversion Factor 1 mi/1.6 km Use this factor to convert kilometers per hour to miles per hour.

0.4 km/1 h * 1mi/1.6 km
▼
Simplify
0.4 km * 1mi/1 h* 1.6 km

Cross out common units

0.4 km * 1mi/1 h* 1.6 km

Cancel out common units

0.4 * 1mi/1 h* 1.6
0.4 mi/1.6 h
0.25 mi/1 h
0.25 mi/1 h
Emily's car goes 0.25 miles per hour. This is faster than Ignacio's car!

ccc Emily's Car & & Ignacio's Car 0.25 mph & > & 0.2 mph



Convert Measurement Units
Exercise 2.1
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