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Scientific Method Logical approach to solving problems Observing is the use of the senses to obtain information. Data may be Qualitative (descriptive): Quality Quantitative (numerical): Quantity System is a specific…during the experiment. You are study the reactions in a test tube: The test tube is the System

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Hypotheses Scientist make generalization based on data This is a Hypothesis If-Then statements Testing Hypotheses Controls: conditions constant Variables: conditions that change

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Strength of hypothesis Repetition and Replication Repetition : experiment will give the same results when it is performed under the same conditions. Replication is the idea that experiments should be reproducible by other scientists.

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Scientific method continued A theory is a broad generalization that explains a body of facts or phenomena. example: atomic theory, collision theory A model : it is often an explanation of how phenomena occur and how data or events are related Example: atomic model of matter

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Scientific Method

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Chapter 2 © Houghton Mifflin Harcourt Publishing Company SI Measurement Scientists all over the world have agreed on a single measurement system called Le Système International d’Unités, abbreviated SI. Section 2 Units of Measurement SI has seven base units most other units are derived from these seven

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Chapter 2 © Houghton Mifflin Harcourt Publishing Company SI Base Units Section 2 Units of Measurement

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Chapter 2 © Houghton Mifflin Harcourt Publishing Company SI Base Units Mass Mass is a measure of the quantity of matter. The SI standard unit for mass is the kilogram. Weight is a measure of the gravitational pull on matter. Mass does not depend on gravity. Section 2 Units of Measurement

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Chapter 2 © Houghton Mifflin Harcourt Publishing Company SI Base Units Length Length is a measure of distance. The SI standard for length is the meter. The kilometer, km, is used to express longer distances The centimeter, cm, is used to express shorter distances Section 2 Units of Measurement

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Chapter 2 © Houghton Mifflin Harcourt Publishing Company Derived SI Units Combinations of SI base units form derived units. pressure is measured in kg/ms 2, or pascals Section 2 Units of Measurement

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Chapter 2 © Houghton Mifflin Harcourt Publishing Company Derived SI Units, continued Volume Volume is the amount of space occupied by an object. The derived SI unit is cubic meters, m 3 The cubic centimeter, cm 3, is often used The liter, L, is a non-SI unit 1 L = 1000 cm 3 1 mL = 1 cm 3 Section 2 Units of Measurement

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Chapter 2 © Houghton Mifflin Harcourt Publishing Company Derived SI Units, continued Density Density is the ratio of mass to volume, or mass divided by volume. Section 2 Units of Measurement The derived SI unit is kilograms per cubic meter, kg/m 3 g/cm 3 or g/mL are also used Density is a characteristic physical property of a substance.

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Chapter 2 © Houghton Mifflin Harcourt Publishing Company Derived SI Units, continued Density Density can be used as one property to help identify a substance Section 2 Units of Measurement

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Chapter 2 © Houghton Mifflin Harcourt Publishing Company Sample Problem A A sample of aluminum metal has a mass of 8.4 g. The volume of the sample is 3.1 cm 3. Calculate the density of aluminum. Section 2 Units of Measurement Derived SI Units, continued

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Chapter 2 © Houghton Mifflin Harcourt Publishing Company Derived SI Units, continued Sample Problem A Solution Given: mass (m) = 8.4 g volume (V) = 3.1 cm 3 Unknown: density (D) Solution: Section 2 Units of Measurement

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Chapter 2 © Houghton Mifflin Harcourt Publishing Company Conversion Factors A conversion factor is a ratio derived from the equality between two different units that can be used to convert from one unit to the other. example: How quarters and dollars are related Section 2 Units of Measurement

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Accuracy & Precision in Measurement

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Accuracy & Precision Accuracy: How close you are to the actual value Depends on the person measuring Calculated by the formula: % Error = (YV – AV) x 100 ÷ AV Where: YV is YOUR measured Value & AV is the Accepted Value Precision: How finely tuned your measurements are or how close they can be to each other Depends on the measuring tool Determined by the number of significant digits

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Accuracy & Precision Accuracy & Precision may be demonstrated by shooting at a target. Accuracy is represented by hitting the bulls eye (the accepted value) Precision is represented by a tight grouping of shots (they are finely tuned)

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Accuracy & Precision Precision without Accuracy No Precision & No Accuracy Accuracy without Precision

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Accuracy - Calculating % Error How Close Are You to the Accepted Value (Bull’s Eye)

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Accuracy - Calculating % Error If a student measured the room width at 8.46 m and the accepted value was 9.45 m what was their accuracy? Using the formula: % error = (YV – AV) x 100 ÷ AV Where YV is the student’s measured value & AV is the accepted value

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Accuracy - Calculating % Error Since YV = 8.46 m, AV = 9.45 m % Error = (8.46 m – 9.45 m) x 100 ÷ 9.45 m = 0.99 m x 100 ÷ 9.45 m = 99 m ÷ 9.45 m = 10.5 % Note that the meter unit cancels during the division & the unit is %. The (-) shows that YV was low The student was off by almost 11% & must remeasure Acceptable % error is within 5%

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remeasure -5% 5% remeasure Acceptable error is +/- 5% Values from –5% up to 5% are acceptable Values less than –5% or greater than 5% must be remeasured

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Significant Digits How to Check a Measurement for Precision

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Significant Figures “Sig Figs” are the actual numbers used to represent a measurement read from an Instrument. Numbers that are considered significant are all of the numbers that can be directly read from the numbers on an instrument plus one estimated. The estimated digit will always be the last digit.

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“IF DOT, THEN RIGHT; IF NOT THEN LEFT” Which digits are Significant? All non-zero (1-9) digits are significant. Some zeros are Significant, and some are not! To determine if zeros are significant, use this simple saying that allows you to draw an arrow to “cross out” zeroes that are ___NOT________ significant. The arrow stops when it reaches a nonzero digit.

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“IF DOT, THEN RIGHT; IF NOT THEN LEFT” How many significant digits are in 0.090? There is a decimal, so the arrow starts outside of the number and points to the right. The first nonzero digit the arrow reaches is a 9, making it, and any digit to the right of it significant. Therefore, there are ___two____ sig figs in 0.090.

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“IF DOT, THEN RIGHT; IF NOT THEN LEFT” How many sig figs are in the number 20400? There is no decimal, so the arrow starts outside of the number and points Left. The first nonzero the digit the arrow reaches is the 4, making it and any digit to the left of it significant Therefore, there are 3 sig figs in the number 20,400

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“IF DOT, THEN RIGHT; IF NOT THEN LEFT” Identify the number of significant figures in the flowing values: 3.909 3,450,000 8.880 0.0670 1,367 0.0002

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More Practice with sig figs How many sig figs? 4,500,0001.3 x 10 2 71.29509.00900121.00 2000.1004 4 3 2 4 34 2 5

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Calculations with Measurements using Significant Figures When adding or subtracting the answer should be rounded to the fewest decimal places. When multiplying or dividing, the answer must have the least number of sig figs. 121.34 g + 1.562 g =122.902 g → 122.90 31.6 m → 32 m5.0 m x 6.32 m =

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What about the rounding? When items have been rounded instead of measured, or for an exactly defined quantity, there are considered to be an Infinite number of sig figs

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Significant Digits & Precision How many digits are there in the measurement? All of these digits are significant There are 3 sig figs. Length of Bar = 3.23 cm cm 12340 What is the length of the bar?

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