Methods of Science. Measurement and Scientific Tools

Transcription

1 Methods of Science Measurement and Scientific Tools Description and Explanation Imagine that a scientist is observing an erupting volcano. He describes in his journal that the flowing lava is bright red with a black crust, and it has a temperature of about 630 C. A description is a spoken or written summary of observations. There are two types of descriptions. A qualitative description, such as bright red, uses senses (sight, sound, smell, touch, taste) to describe an observation. A quantitative description, such as 630 C, uses numbers and measurements to describe an observation. Later, the scientist might explain his observations. An explanation is an interpretation of observations. Because the lava was bright red and about 630 C, the scientist might explain the lava is cooling, and the volcano did not recently erupt. The International System of Units At one time, scientists in different parts of the world used different units of measurement. This made sharing scientific information difficult. Imagine the confusion when a British scientist measured weight in pounds-force and a Japanese scientist measured in momme (MOM ee). In 1960, scientists adopted a new system of measurement. The International System of Units (SI) is the internationally accepted system for measurement. SI uses standards of measurement, called base units, shown in the table below. SI Base Units Quantity Measured Unit Symbol Length meter m Mass kilogram kg Time second s Electric current ampere A Temperature Kelvin K Amount of substance mole mol Intensity of light candela cd

2 SI Unit Prefixes In addition to base units, SI uses prefixes to identify the size of the unit, as shown in the table below. Prefixes are used to indicate a fraction of ten or a multiple of ten. In other words, each unit is either ten times smaller than the next larger unit or ten times larger than the next smaller unit. For example, the prefix deci means 10-1, or 1/10. A decimeter is 1/10 of a meter. The prefix kilo means 10 3, or 1,000. A kilometer is 1,000 m. Prefix Prefixes Meaning Mega- (M) 1,000,000 (10 6 ) Kilo- (k) 1,000 (10 3 ) Hecto- (h) 100 (10 2 ) Deka- (da) 10 (10 1 ) Deci- (d) 0.1 (10-1 ) Centi- (c) 0.01 (10-2 ) Milli- (m) (10-3 ) Micro- (μ) (10-6 ) Converting Between SI Units Because SI is based on ten, it is easy to convert from one SI unit to another. To convert SI units, you must multiply or divide by a factor of ten. You also can use proportions, as shown in the Math Skills activity on this page. Measurement and Uncertainty Have you ever measured an object, such as a paper clip? The tools used to take measurements can limit the accuracy of the measurements. All measurements have some uncertainty. If you measured a paper clip with a ruler divided into centimeters, you would know that the paper clip is between 4 cm and 5 cm, because only whole centimeters are shown. You might guess that the paper clip is 4.5 cm long. With a ruler that has measurements divided into millimeters, you could say with more certainty that the paper clip is about 4.7 cm long. This measurement is more accurate than the first measurement.

3 Significant Digits and Rounding Because scientists duplicate each other s work, they must record numbers with the same degree of precision as the original data. Significant digits allow scientists to do this. Significant digits are the number of digits in a measurement that you know with a certain degree of reliability. In order to achieve the same degree of precision as a previous measurement, it often is necessary to round a measurement to a certain number of significant digits. Suppose you need to round the number below to four significant digits. 1, g To round to four significant digits, you need to round the 8. If the digit to the right of the 8 is 0, 1, 2, 3, or 4, the digit being rounded (8) remains the same. If the digit to the right of the 8 is 5, 6, 7, 8, or 9, the digit being rounded (8) increases by one. The rounded number is 1,349 g. What if you need to round 1, g to two significant digits? You would look at the number to the right of the 3 to determine how to round. 1, rounded to two significant digits would be 1,300 g. The 4 and 8 become zeros. The table below shows some rules for expressing and determining significant digits. Significant Digits Rules 1. All nonzero numbers are significant. 2. Zeros between nonzero digits are significant. 3. One or more final zeros used after the decimal point are significant. 4. Zeros used solely for spacing the decimal point are NOT significant. The zeros only indicate the position of the decimal point. Note: The bold numbers in the examples are the significant digits. Number Significant Digits Applied Rules , , , 3 1, , , , , 4 50, , 2, 4

4 Mean, Median, Mode, and Range A rain gauge measures the amount of rain that falls on a location over a period of time. A rain gauge can be used to collect data in scientific investigations, such as the data shown in the table below. Scientists often need to analyze their data to obtain information. Four values often used when analyzing numbers are median, mean, mode, and range. Month Order Rainfall Data Numerical Order January 7.11 cm 1.47 cm February cm 7.11 cm March 9.58 cm 7.11 cm April 8.18 cm 8.18 cm May 7.11 cm 8.84 cm June 1.47 cm 9.58 cm July cm cm August 8.84 cm cm Median The median is the middle number in a data set when the data are arranged in numerical order. The rainfall data are listed in numerical order in the right column of the table above. The items in bold are the two middle numbers. If you have an even number of data items, add the two middle numbers together and divide by two to find the median cm cm median = = 8.51 cm 2 Mean The mean, or average, of a data set is the sum of the numbers in a data set divided by the number of entries in the set. To find the mean, add the numbers in your data set and then divide the total by the number of items in your data set. mean = = (sum of numbers) (number of items) cm 8 months = 9.05 cm month Mode The mode of a data set is the number or item that appears most often. The number 7.11 occurs twice. All other numbers only appear once. mode = 7.11

5 Range The range is the difference between the greatest number and the least number in the data set. range = = Scientific Tools As you engage in scientific inquiry, you will need tools to help you take quantitative measurements. Always follow appropriate safety procedures when using scientific tools. Science Journal Use a science journal to record observations, questions, hypotheses, data, and conclusions from your scientific investigations. A science journal is any notebook that you use to take notes or record information and data while you conduct a scientific investigation. Keep your journal organized so you can find information easily. Write the date whenever you record new information in the journal. Make sure you are recording your data honestly and accurately. Rulers and Metersticks Use rulers and metersticks to measure lengths and distances. The SI unit of measurement for length is the meter (m). For small objects, such as pebbles or seeds, use a metric ruler with centimeter and millimeter markings. To measure larger objects, such as the length of your bedroom, use a meterstick. To measure long distances, such as the distance between cities, use an instrument that measures in kilometers. Be careful when carrying rulers and metersticks, and never point them at anyone. Glassware Use beakers to hold and pour liquids. The lines on a beaker are not very precise measurements, so you should use a graduated cylinder to measure the volume of a liquid. Liquid volume is typically measured in liters (L) or milliliters (ml).

6 Triple-Beam Balance Use a triple-beam balance to measure the mass of an object. The mass of a small object is measured in grams. The mass of a large object is usually measured in kilograms. Triple-beam balances are instruments that require some care during use. Follow your teacher s instructions so that you do not damage the instrument. Digital balances also might be used. Thermometer Use a thermometer to measure the temperature of a substance. Although Kelvin is the SI unit for temperature, you will use a thermometer to measure temperature in degrees Celsius ( C). To use a thermometer, place a room-temperature thermometer into the substance for which you want to measure temperature. Do not let the thermometer touch the bottom of the container that holds the substance or you will get an inaccurate reading. When you finish, remember to place your thermometer in a secure place. Do not lay it on a table, because it can roll off the table. Never use a thermometer as a stirring rod. Computers and the Internet Use a computer to collect, organize, and store information about a research topic or scientific investigation. Computers are useful tools to scientists for several reasons. Scientists use computers to record and analyze data and to research new information. They also can quickly share their results with others worldwide over the Internet.

7 Tools Used by Earth Scientists Binoculars Binoculars are instruments that enable people to view faraway objects more clearly. Earth scientists use them to view distant landforms, animals, or even incoming weather. Compass A compass is an instrument that shows magnetic north. Earth scientists use compasses to navigate when they are in the field and to determine the direction of distant landforms or other natural objects. Wind Vane and Anemometer A wind vane is a device, often attached to the roof of a building, that rotates to show the direction of the wind. An anemometer, or wind-speed gauge, is used to measure the speed and the force of wind. Streak Plate A streak plate is a piece of hard, unglazed porcelain that helps you identify minerals. When you scrape a mineral along a streak plate, the mineral leaves behind powdery marks. The color of the mark is the mineral s streak.