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## Ch. 2 Sect. 2

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**Ch. 2 Sect. 2**Measurement and uncertainties**Precision and Accuracy**• Precision • Describes degree of exactness of a measurement • Which is more precise • Measurement A = 18.8 [+/-] 0.3 cm • Measurement B = 19.0 [+/-] 0.2 cm • Measurement C = 18.3 [+/-] 0.1 cm • Measurement C is more precise**Precision and Accuracy**• Accuracy • Describes how well results of an experiment agree with standard value • If block from previous data was 19.0 cm, which measurement was most/least accurate? • Measurement B was most accurate • Measurement C was less accurate**Techniques of taking measurements**• Make sure that your angle when taking a measurement is correct • Volume of a liquid • Length on a meter-stick • Parallax • Apparent shift in position of an object when veiwed at different angles**Significant Digits/Figures**• Valid digits in a measurement**Measurement**• Object A • Ruler 1- 6.80 cm • The “0” is the estimate or uncertain digit • There are 3 sig figs in this measurement • Ruler 2 – 6.8 cm • The “8” is your uncertain digit • There are only 2 sig figs • Which is more precise? • Ruler 1**Which Zero’s are significant?**• If you answered measurement “0.0680 m” you would still only have 3 sig figs • First two zeros only locate the decimal (place holders) and aren’t significant • The last zero is the estimated digit • 186 000 m • 6 is the estimated digit while 0’s locate decimal • 3 sig figs • 186.000 km?? • 6 sig figs**Rules for Sig Figs**• 1) Non zero digits are always significant • 2) All final zeros after the decimal point are significant • 3) Zeros between two other sig digits are significant • 4) Zeros that are used as place holders are not significant**Practice Problems**• 2804 m • 2.84 km • 0.0029 m • 0.003068 m • 4.6 x 105 m • 4.06 x 10-5 m • 75.00 m • 0.007060 kg • 1.0080 x 107 km**Practice Problems**• 2804 m 4 • 2.84 km 3 • 0.0029 m 2 • 0.003068 m 4 • 4.6 x 105 m 2 • 4.06 x 10-5 m 3 • 75.00 m 4 • 0.007060 kg 4 • 1.0080 x 107 km 5**Add and Subtract with sig figs**• Important to remember that the result can never be more precise than the least precise measurement • When (+) or (-), first perform the operation then round to the least precise value • 24.686 m • 2.343 m • +3.21 m • =30.239 30.24 m**Multiply or divide**• After (x) or (/) calculation, note the factor with least number of sig figs and round product or quotient to that number of digits • 3.22 cm x 2.1 cm • = 6.762 6.8 cm2