Rational Expressions - Dimensional Analysis

Transcription

1 7.8 Rational Expressions - Dimensional Analysis One application of rational expressions deals with converting units. When we convert units of measure we can do so by multiplying several fractions together in a process known as dimensional analysis. The trick will be to decide what fractions to multiply. When multiplying, if we multiply by, the value of the expression does not change. One written as a fraction can look like many different things as long as the numerator and denominator are identical in value. Notice the numerator and denominator are not identical in appearance, but rather identical in value. Below are several fractions, each equal to one where numerator and denominator are identical in value. = 4 4 = 4 = 00cm m = lb 6oz = hr 60 min = 60 min hr The last few fractions that include units are called conversion factors. We can make a conversion factor out of any two measurements that represent the same distance. For example, mile = 580 feet. We could then make a conversion mi 580ft factor because both values are the same, the fraction is still equal to one. Similarly we could make a conversion factor 580ft. The trick for conversions will mi be to use the correct fractions. The idea behind dimensional analysis is we we multiply by a fraction in such a way that the units we don t want will divide out of the problem. We found out when multiplying rational expressions that if a variable appears in the numerator and denominator we can divide it out of the expression. It is the same with units. Consider the following conversion. Example. 7.7 ( miles to feet Write 7.7 miles asafraction, put it over 7.7mi To divide out the miles we need miles in the denominator ( ( 7.7mi??ft We are converting to feet, so this will go in the numerator??mi ( ( 7.7mi 580ft Fill in the relationship described above, mile = 580 feet mi ( ( ft Divide out the miles and multiply across 9, 7.6ft Our Solution In the previous example, we had to use the conversion factor 580ft so the miles mi would divide out. If we had used mi we would not have been able to divide out 580ft the miles. This is why when doing dimensional analysis it is very important to use units in the set-up of the problem, so we know how to correctly set up the conversion factor.

2 Example. If pound = 6 ounces, how many pounds 45 ounces? ( 45oz Write 45 asafraction, put it over ( 45 ( 45oz ( 45oz (?? lbs??oz ( lbs 6oz ( lbs 45 lbs = 6 6 To divide out oz, lbs Our Solution put it in the denominator and lbs in numerator Fill in the given relationship, pound=6 ounces Divide out oz, multiply across. Divide result The same process can be used to convert problems with several units in them. Consider the following example. Example. A student averaged 45 miles per hour on a trip. What was the student s speed in feet per second? ( 45mi per is the fraction bar, put hr in denominator hr ( 45mi hr ( 45 ( 45mi hr ( 580ft mi ( ( 580ft hr mi 600 sec ( ( 580ft 600 sec Toclearmitheymustgoindenominatorandbecomeft Toclearhrtheymustgoinnumeratorandbecomesec Divide out mi and hr. Multiply across 7600ft 600 sec Divide numbers 66 ft per sec Our Solution If the units are two-dimensional (such as square inches - in or thre-dimensional (such as cubic feet - ft we will need to put the same exponent on the conversion factor. So if we are converting square inches (in to square ft (ft, the conversion

3 factor would be squared, the convesion factor. Example 4. ( ft. in Similarly if the units are cubed, we will cube Convert 8 cubic feet to yd ( 8ft Write8ft as fraction, put it over To clear ft, put them in denominator, yard in numerator ( 8 ( ( 8ft??yd Because the units are cubed,??ft we cube the conversion factor ( ( 8ft yd Evaluate exponent, cubing all numbers and units ft ( ( 8ft yd ( yd 7 7ft = 8yd 7 Divide out ft Multiply across and divide yd Our Solution When calculating area or volume, be sure to use the units and multiply them as well. Example 5. A room is 0 ft by ft. How many square yards are in the room? A = lw =(0ft( ft=0ft ( 0ft Multiply length by width, also multiply units Write area asafraction, put it over ( ( 0ft??yd Put ft in denominator to clear,??ft square conversion factor ( ( 0ft yd Evaluate exponent, squaring all numbers and units ft ( ( 0ft yd 9ft Divide out ft

4 ( 0 ( yd 9 = 0yd 9 Multiply across and divide.yd Our solution To focus on the process of conversions, a conversion sheet has been included at the end of this lesson which includes several conversion factors for length, volume, mass and time in both English and Metric units. The process of dimensional analysis can be used to convert other types of units as well. If we can identify relationships that represent the same value we can make them into a conversion factor. Example 6. A child is perscribed a dosage of mg of a certain drug and is allowed to refill his perscription twice. If a there are 60 tablets in a perscription, and each tablet has 4 mg, how many doses are in the perscriptions (original + refills? Convert Rx to doses Idenfity what problem is asking Rx=60 tab, tab =4mg, dose = mg Idenfity given conversion factors ( Rx ( Rx ( Rx ( ( 60 tab 4mg Rx tab ( ( Rx ( 60 tab Rx ( ( 60 tab 4mg Rx tab ( 60 ( 4 ( dose mg ( dose 70 dose WriteRx as fraction, put over Convert Rx to tab, put Rx in denoimnator Convert tab to mg, put tab in denominator Convert mg to dose, put mg in denominator Divide out Rx, tab, and mg, multiply across Divide 60 doses Our Solution Beginning and Intermediate Algebra by Tyler Wallace is licensed under a Creative Commons Attribution.0 Unported License. ( 4

5 7.8 Practice - Dimensional Analysis Use dimensional analysis to convert the following: 7 mi. to yards 4 oz. to tons. mg to grams 4.5 km to centimeters 5 9,800,000 mm (milimeters to miles ft to square yards 7 45,000 m to squre kilometers 8 8 km to square feet km to cubic meters in to cubic centimeters 5,500 cm to cubic yards.5 mph (miles per hour to feet per second 85 yd. per min. to miles per hour 4 5 ft/s (feet per second to miles per hour 5 48 mph to meters per second 6 86,000 mph to kilometers per year T/yd (tons per square yard to pounds per square inch 8 6 ft/s to kilometers per hour squared Use dimensional analysis to solve the following: 9 On a recent trip, Jan traveled 60 miles using 8 gallons of gas. How many miles per -gallon did she travel? How many yards per -ounce? 0 A chair lift at the Divide ski resort in Cold Springs, WY is 4806 feet long and takes 9 minutes. What is the average speed in miles per hour? How many feet 5

6 per second does the lift travel? A certain laser printer can print pages per minute. Determine this printer s output in pages per day, and reams per month. ( ream = 5000 pages An average human heart beats 60 times per minute. If an average person lives to the age of 75, how many times does the average heart beat in a lifetime? Blood sugar levels are measured in miligrams of gluclose per deciliter of blood volume. If a person s blood sugar level measured 8 mg/dl, how much is this in grams per liter? 4 You are buying carpet to cover a room that measures 8 ft by 40 ft. The carpet cost S8 per square yard. How much will the carpet cost? 5 A car travels 4 miles in 5 minutes. How fast is it going in miles per hour? in meters per second? 6 A cargo container is 50 ft long, 0 ft wide, and 8 ft tall. Find its volume in cubic yards and cubic meters. 7 A local zoning ordinance says that a house s footprint (area of its ground floor cannot occupy more than of the lot it is built on. Suppose you own a 4 -acre lot, what is teh maximum allowed footprint for your house in square feet? in square inches? ( acre = 4560 ft 8 Computer memory is measured in units of bytes, where one byte is enough memory to store one character (a letter in the alphabet or a number. How many typical pages of text can be stored on a 700-megabyte compact disc? Assume that one typical page of text contains 000 characters. ( megabyte =,000,000 bytes 9 In April 996, the Department of the Interior released a spike flood from the Glen Canyon Dam on the Colorado River. Its purpose was to restore the river and the habitants along its bank. The release from teh dam lasted a week at a rate of 5,800 cubic feet of water per second. About how much water was released during the -week flood? 0 The largest single rough diamond ever found, the Cullinan diamond, weighed 06 carats; how much does the diamond weigh in miligrams? in pounds? ( carat - 0. grams Beginning and Intermediate Algebra by Tyler Wallace is licensed under a Creative Commons Attribution.0 Unported License. ( 6

7 7.8 Answers - Dimensional Analysis 0 yd T 0.0 g 4 5,000 cm 5 6. mi yd km 8 86,067,00 ft 9 6,500,000 m cm yd 5. ft/sec 6. mph mi/hr 5 m/s 6,6,69,600 km/yr 7.6 lb/in 8 6,9.5 km/hr 9.5 mph; 447 yd/oz mi/hr 780 pages/day; 0.4 reams/month,65,00,000 beats/lifetime.8 g/l 4 S mph; 5 m/s yd 7 60 ft 8 50,000 pages 9 5,60,840,000 ft /week 0 6,00 mg; 68 lb Beginning and Intermediate Algebra by Tyler Wallace is licensed under a Creative Commons Attribution.0 Unported License. ( 7