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| 13 Theory slides |
| 12 Exercises - Grade E - A |
| Each lesson is meant to take 1-2 classroom sessions |
Here are a few recommended readings before getting started with this lesson.
The combination of base units creates new units of measure. Some of these combinations have their own names. Using the applet below, units like m/s^2 can be created following these two steps.
To have a clean start, press the Reset
button. Any of the units inside the boxes can be drag either to the numerator or denominator of the fraction at the right. Build new units and find out if they have a particular name.
One rainy morning, Jordan saw lightning from her bedroom window. After a few seconds of time had passed, she heard the sound of thunder, which shook the photos on her desk. Could Jordan know the distance from her house to the lightning struck? What information does she need?
In the sentence Voyager 1 travels at 61 000 kilometers per hour,
both a unit of time and a unit of length are involved. In this case, the unit of length is distance traveled, and the unit of time is the amount of time it takes for the item to travel the given distance. Combining these units leads to a new type of unit that describes how fast an object is moving.
The rate or speed of a moving object, denoted by r, measures how fast the object is moving — that is, the rate of motion of the object. It is calculated by dividing the distance traveled by the amount of time spent traveling.
Speed = Distance traveled/Time interval
From the above, three formulas can be derived relating speed, distance, and time. The triangular diagram below helps to set each formula.
Multiply the speed of the train by the time it took to go from one city to the other.
r= 47.5 mi/h, t= 4 h
Rewrite 47.5 mi/h as 47.5 mih
Cross out common factors
Cancel out common factors
a/1=a
Multiply
Sometimes, two quantities can be given using different units of measure. However, to operate with the quantities, they need to have the same unit. Consider the diagram below, where d_1 is given in meters, d_2 is given in miles, and the total distance d_1+d_2 is required to be calculated.
In such cases, one of the quantities needs to be converted into an equivalent quantity using the other quantity's unit of measure. For example, d_2 can be converted from miles to meters. In such instances, the conversion factor plays an important role.
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. Examine how to convert 2 hours to minutes using the above conversion factor.
Given Quantity | Conversion | Result |
---|---|---|
2 hours | 2 hours * 60 minutes/1 hour | 120 minutes |
Although the final result is in minutes, both quantities represent the same amount of time. Note that the opposite conversion, from minutes to hours, has a conversion factor of 1 hour60 minutes. If the task was to convert 120 minutes to hours, 120 minutes would be multiplied by this conversion factor.
Given Quantity | Conversion | Result |
---|---|---|
120 minutes | 120 minutes * 1 hour/60 minutes | 2 hours |
As shown in the examples above, 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 desired units. In the table, some common conversion factors are used to convert the given measures.
Given Quantity | Conversion | Result |
---|---|---|
3 pounds | 3 pounds * 16 ounces/1 pound | 48 ounces |
160 ounces | 160 ounces * 1 pound/16 ounces | 10 pounds |
1 mile | 1 mile * 1760 yards/1 mile | 1760 yards |
Some common conversions involve distance, mass, area, volume, time, and temperature.
The numerator and denominator of the conversion factor represent the same quantity. That means 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.
When converting from one unit to another, the desired unit needs to be in the numerator of the conversion factor while the given unit needs to be in the denominator. That way when the quantity is multiplied by the conversion factor, the given unit will cancel out and the desired unit will remain.
Keep in mind that, despite the given quantity and the new quantity have different values, they represent the same amount.
To convert d_2 from miles to meters, use the fact that 1 mile is the same as 1609.34 meters. 0.25 mi * 1609.34 m/1 mi = 402.335 m It has been shown that 0.25 miles is the same as 402.335 meters. Now that both d_1 and d_2 have the same unit of measure, d_1+d_2 can be found.
d_1+ d_2 &= 300 m + 402.335 m &= 702.335 mTearrik is studying about the theory of baseball during study hall. He reads that on a baseball field, the distance from the pitcher's mound to home plate is 14.8 meters.
At his fastest, Tearrik can throw a fastball at 91 miles per hour. How many seconds does it take for the ball to reach home plate? Round the answer to two decimal places.
Use the fact that 1 mile is 1609.34 meters and 1 hour is 3600 seconds.
From the given information, speed and distance are given and time is required.
Multiply by 1609.34 m/mi
Write as a fraction
Cross out common factors
Cancel out common factors
Multiply
Multiply by 1 h/3600 s
Write as a fraction
Cross out common factors
Cancel out common factors
Calculate quotient
Round to 2 decimal place(s)
d= 14.8 m, r= 40.68 m/s
a/b/c= a * c/b
Cross out common factors
Calculate quotient
Round to 2 decimal place(s)
As these examples show, speed combines two different base units, distance and time, in a quotient. Similarly, different combinations of base units can lead to new units, which are called derived units.
Derived units are units of measurement formed by multiplying or dividing base units in different combinations.
Quantity (Symbol) | Expression | Base Units | Unit Symbol (Name) |
---|---|---|---|
Area (A) | length * width | m^2 | m^2 |
Volume (V) | area * height | m^3 | m^3 |
Speed (r) | distance/time | .m /s. | .m /s. |
Acceleration (a) | speed/time | .m /s^2. | .m /s^2. |
Density ( ) | mass/length^3 | .kg /m^3. | .kg /m^3. |
Force (F) | mass * acceleration | .kg* m /s^2. | N (newton) |
Pressure (P) | force/area | .N /m^2. | Pa (pascal) |
Energy (E) | force * distance | N*m | J (joule) |
Power (P) | force* distance/time | .J /s. | W (watt) |
Frequency (F) | 1/time | .1 /s. | Hz (hertz) |
One common example of a derived unit is the acceleration of an object, found by dividing the change in speed by the change in time. The change in speed is measured as the difference between the final and initial speeds.
Acceleration = Final speed-Initial speed/Time taken
When crossing the starting line, the speed of a Formula 1 car is 50 meters per second. After 6 seconds, its speed is 86 meters per second. Find the acceleration rate, in meters per second squared, of the car in that time interval.
The acceleration rate equals the change in speed divided by the time taken. The change in speed is the final speed minus the initial speed.
Final speed= 86 m/s, Initial speed= 50 m/s
Subtract terms
Change in speed= 36 m/s, Time taken= 6 s
Write as a fraction
a/c/b= a/b* c
Multiply
Calculate quotient
From the derived units table, the base units for newtons are kilograms multiplied by meters per second squared. Therefore, the mass of the ball needs to be converted from grams to kilograms. Acceleration equals the change in speed divided by the change in time.
Multiply by 1 kg/1000 g
Cross out common factors
Cancel out common factors
Multiply
Calculate quotient
r= 49 m/s, t= 4 s
Write as a fraction
a/c/b= a/b* c
Multiply
Calculate quotient
m= 0.145 kg, a= 12.25 m/s^2
Multiply
Round to 2 decimal place(s)
kg*m/s^2= N
Saturday morning before baseball practice, Tearrik went out for a jog beginning at 10:00 A.M. He was 1500 meters away from his starting location at 10:10 A.M. Over the next 5 minutes he jogged another 500 meters. Tearrik then jogged another 1120 meters before finally stopping at 10:22 A.M. Make a graph representing the time and Tearrik's speed during his jog.
Place time on the horizontal axis and speed on the vertical axis. Find the speed in each time interval by dividing the distance covered by the time interval.
Start by organizing the given information in a table. Note that information for three time intervals is given.
Start Time | End Time | Time Interval (minutes) | Distance Covered (meters) |
---|---|---|---|
10:00 A.M. | 10:10 A.M. | 10 | 1500 |
10:10 A.M. | 10:15 A.M. | 5 | 500 |
10:15 A.M. | 10:22 A.M. | 7 | 1120 |
To find the speed in each interval, divide the distance by the time interval.
Time Interval (minutes) | Distance Covered (meters) | Speed (meters per minute) |
---|---|---|
10 | 1500 | 1500/10 = 150 |
5 | 500 | 500/5 = 100 |
7 | 1120 | 1120/7 = 160 |
Next, place time on the horizontal axis and speed on the vertical axis. Each number to the right of the origin corresponds to the minutes after 10:00 A.M. Based on the table, it is convenient to divide the horizontal axis into 5-minute intervals and the vertical axis into intervals of 50 meters per minute.
Next, translate the information from the table to the graph.
With all this new information learned, the time it takes Voyager 1 to reach Proxima Centauri can now be calculated. Recall that Voyager 1 travels at a constant speed of 61 000 kilometers per hour and that the distance from Earth to Proxima Centauri is 4.24 light years, which is the same as 4.014* 10^(13) kilometers. Speed &= 61 000 km/h Distance &= 4.014* 10^(13) km To determine when Voyager 1 will reach Proxima Centauri, use the formula below.
Time= Distance/Speed
d= 4.014* 10^(13) km, r= 61 000 km/h
a/b/c= a * c/b
Cross out common factors
Cancel out common factors
Multiply by 1 day/24 h*1 year/365 days
Cross out common factors
Multiply fractions
Multiply
Calculate quotient
Round to nearest integer
Carry out the following conversions.
Converting between feet (ft) and inches (in.) will involve using a conversion factor. Since 12 inches equals 1 foot, the conversion factor becomes the following. 12in./1 ft Multiplying 7 ft by this conversion factor will convert it to inches.
By adding the original 4 inches to 84 inches, the length can be converted to inches only. 7 ft 4 in. =84+4= 88in.
Converting from kilograms (kg) to pounds (lbs) will involve using a conversion factor. As 1 pound is approximately the same as 0.454 kilograms, the conversion factor becomes the following. 1lb/0.454kg Multiplying 908 kilograms by this conversion factor will convert it to pounds.
Converting from hours (h) to minutes (min) involves using a conversion factor. Because in 1 hour there are 60 minutes, the conversion factor becomes the following.
60min/1h
Multiplying 3.5 hours by this conversion factor will convert it to minutes.
To convert centimeters to meters, a conversion factor is required. Since 100 centimeters equals 1 meter, the conversion factor becomes the following.
1 m/100 cm
Multiplying 200 centimeters by this conversion factor will convert it to meters.
Which of the recipes requires the least amount of flour per loaf of bread?
By determining the unit rate of each recipe, the recipe that requires the least amount of flour per loaf of bread can be determined. Unit Rate: # cups/1loaf Let's calculate the unit rate for each recipe.
Recipe | # cups/# loaves | # cups/1 loaf |
---|---|---|
A | 1 cup/5 loaves | 0.2 cups/1 loaf |
B | \dfrac {3 \text{ cups}}{12 \text{ loaves}} | \dfrac{0.25 \text{ cups}} {1 \text{ loaf}} |
C | 6 cups/20 loaves | 0.3 cups/1 loaf |
D | 4 cups/21 loaves | ≈0.19 cups/1 loaf |
E | 8 cups/32 loaves | 0.25 cups/1 loaf |
Recipe D requires the least amount of flour per loaf.
When the distance covered by an object and the time it takes are known, the speed can be calculated using the following formula. Distance=Speed * Time By substituting the time and the distance between the two cities, we can determine the speed at which the siblings need to drive. According to their dad, the distance between New York and Los Angeles is 3200 miles and took him 50 hours.
The siblings must drive at an average speed of 64 miles per hour to reach New York 50 hours after leaving Los Angeles.
We know that 1 mile is about 1.609 kilometers. 1 mi ≈ 1.609 km For a conversion factor to work, we need to position these units so that the unit we do not want will cancel out. Since the original quantity is in kilometers, we should place kilometers in the denominator of our conversion factor. 1 mi/1.609 km When the original quantity is multiplied by the conversion factor, this allows for kilometers to cancel out. The miles unit remains. \begin{aligned} \left( ^{\,\text{Original}}_\text{Quantity} \right) {\color{#FF0000}{\cancel{{\color{#0000FF}{\text{km}}}}}} \cdot \dfrac{1 {\color{#009600}{\ \text{mi}}}}{1.609 \ {\color{#FF0000}{\cancel{{\color{#0000FF}{\text{km}}}}}}} = \underbrace{\left(\dfrac{^{\,\text{Original}}_\text{Quantity}}{1.609} \right)}_{\stackrel{\scriptsize{\text{converted}}}{\text{quantity}}} \ {\color{#009600}{\text{mi} }} \end{aligned} Therefore, the desired resulting units, miles, will be in the numerator of the conversion factor.
To determine the runner with the fastest average speed, we should calculate the average speed of each person. To do so, we will use the following formula. Speed=Distance/Time By substituting distance and time, we can determine the speed of each runner.
Runner | Distance & Time | Distance/Time | Speed |
---|---|---|---|
Diego | d= 15 t= 3 |
15/3 | 5 km/h |
LaShay | d= 8 t= 2 |
8/2 | 4 km/h |
Kriz | d= 9 t= 3 |
9/3 | 3 km/h |
As we can observe, Diego was the fastest, Kriz was the slowest, and LaShay was in the middle.