Physics                Unit A                 Day 2 (Aug 26, 2008)

 

Objectives         

¨      Define SI standards of measurement.

¨      Use common metric prefixes.

¨      Distinguish between accuracy and precision.

¨      Indicate the precision of measured quantities with significant digits.

¨      Perform operations with significant digits.

 

1.              Homework check and discuss Chp. 2 Supplemental Problems

 

2.              Measurements.

Two basic systems – Imperial or English and Metric or SI System

           

            SI Base Units                                                                  Derived Units

            Length – meter – m                                                         combinations of base units

            Mass – kilogram – kg                                                      e.g. m/s, kg.m/s2

            Time – second – s

            Temperature – Kelvin – K

            Amount of substance – mole – mol

 

3.              SI Prefixes

Used to change SI units by powers of 10. The same prefixes are used for all quantities.

Ex. mm, ms, mg, mmol

 

            Common Prefixes

            G – Giga – 109                M – Mega – 106  k – kilo – 103                 c – centi – 10-2

            mmilli – 10-3  m – micro – 10-6  n – nano – 10-9   

 

Converting Units

Ex. 1                 185 mg              185 mg x    1 g       = 0.185 g

                                                                             1000 mg

Ex. 2                 3.5 x 107 m                                3.5 x 107 m x 1 Mm  = 35 Mm

                                                                                                         1 x 106 m

4.              Measurement Uncertainties

Every measurement is subject to some uncertainty due to the quality of the instrument used and how the instrument was used.

 

Ex. Mass balance Use electronic balance to measure mass of object.

Reading (ex.) 97. 32 g

The precision of the instrument is to nearest 0.01 g (the last place value of the measurement)

 

            Ex. 34.7 km (precise to 0.1 km)                 0.483 mg is precise to 0.001 mg

 

Notice that the last digit is changing slightly. This communicates the certainty of the measurement. We are “certain” of the first three digits, but the last one is estimated (in this case by the balance). The certainty of a measurement is communicated by the number of significant digits. In this case 4 significant digits.

 

The accuracy of a measurement describes how “close” the measured value agrees with an accepted standard.

Ex. The diameter of a bearing is 14. 8 mm Ī 5%

 

Rules of Significant digits (p. 26 text)

1.              Non zero digits are always significant

2.              All final zeros after the decimal point are significant.

3.              Zeros between other significant digits are significant.

4.              Zeros solely used as placeholders are not significant.

 

Ex. 1.560 g – 4 sd           1.506 g – 4 sd                 0.156 g – 3sd      15 600 g – 3 sd

 

 

 

5.         Precision and Certainty Rules

 

            Precision Rule (used for addition and subtraction)

            E.g.                  17. 36 g

                           +         8.1    g

                                    25.46 g   round to 25. 5 g (least place value)

 

            Certainty Rule (used for multiplication and division)

            E.g.                  17.36 m/s x 8.1 s = 141.616 m  round to 140 m (round to least no. of sd)

 

 

6.         Assign.

            Read Chapter 1; Q5,8,10,11,17,18,20,22,25,27,28,29,39,40,42,46,49,51,59,60

            Mastering problems: Review Q83-97