# cm18 - Chapter 3 Scientific measurement Types of...

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Chapter 3 Scientific measurement

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Types of observations Qualitative- descriptive, but not true measurements Hot Large Quantitative- describe with numbers and units 100 ° C 15 meters
Types of observations Scientists prefer Quantitative More precise No bias testable

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How good are the measurements? Scientists use two word to describe how good the measurements are- Accuracy - how close the measurement is to the actual value. Precision - how well can the measurement be repeated.
Differences Accuracy can be true of an individual measurement or the average of several. Precision requires several measurements before anything can be said about it. examples

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Let’s use a golf analogy
Accurate? No Precise? Yes

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Accurate? Yes Precise? Yes
Precise? No Accurate? Maybe?

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Accurate? Yes Precise? We cant say!
In terms of measurement Three students measure the room to be 10.2 m, 10.3 m and 10.4 m across. Were they precise? Were they accurate?

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Error Accepted value – The right answer Based on reliable references Experimental Value- what you get in lab Error = experimental value – accepted value Can be negative
Percent Error error Percent Error = 100% accepted value × Absolute value of error I know that I weigh 150 kg. If I weigh myself and the balance says 165 kg, what is the percent error?

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Significant figures (sig figs) How many numbers mean anything. When we measure something, we can (and do) always estimate between the smallest marks. 2 1 3 4 5
Significant figures (sig figs) The better marks the better we can estimate. Scientist always understand that the last number measured is actually an estimate. 2 1 3 4 5

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Significant figures (sig figs) The measurements we write down tell us about the ruler we measure with The last digit is between the lines What is the smallest mark on the ruler that measures 142.13 cm? 141 142
Significant figures (sig figs) What is the smallest mark on the ruler that measures 142 cm? 100 200 150 250 50

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140 cm? Here there’s a problem is the zero significant or not? 100 200 150 250 50 100 200
140 cm? They needed a set of rules to decide which zeroes count. All other numbers do count . 100 200 150 250 50 100 200

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Which zeros don’t count as sig figs? Those at the end of a number before the decimal point don’t count. 12400 If the number is smaller than one, zeroes before the first number don’t count. 0.045 These zeros are only place holders
figs? Zeros between other sig figs do. 1002 Zeroes at the end of a number after the decimal point do count. 45.8300

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## This document was uploaded on 02/06/2011.

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cm18 - Chapter 3 Scientific measurement Types of...

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