Welcome to Grade 11 Physics SI units and
4204-si_units,_scienctific_notation.ppt
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Welcome to Grade 11 Physics SI units and Scientific Notation Chapter 2
Units of Measurement Measurements involve NUMBER and UNIT Represent a quantity: has magnitude, size, or amount Gram = unit of measurement Mass = quantity
Units of Measurement Scientists around the world agree on one system… International System of Units (le Systeme International d’Unites) SI units Built from seven base units
SI Fundamental or Base Units
Metric prefix Prefixes – powers of ten
Mass Measures quantity of matter SI unit: kilogram, kg ______ kg = _____ g gram used for smaller masses Weight: measure of gravitational pull
Length SI unit: meter, m Longer distances: kilometer, km _______ km = _______ m Shorter distances: centimeter, cm _______ m = ________ cm
Derived Units, like Volume SI unit: m3 A derived unit: combination of base units by multiplying or dividing SI unit for Area: l x w = m x m = m2 Volume: l x w x h = m x m x m = m3 Also: liters (L), mL, dm3 and cm3 1 L = 1 dm3 = 1000mL = 1000 cm3
Derived Units
Scientific Notation Put the numbers in the form a x 10n a has one # to left of decimal If # is bigger than 1 + exponent If # is less than 1 - exponent
Scientific Notation Review: Write in scientific notation 32,700 0.0003412 Change to a decimal 3.901 x 10-6 4.755 x 108
Significant Figures (sig figs) How many numbers mean anything? When we measure, we can (and do) always estimate between the smallest marks.
Significant figures (sig figs) Better marks better estimate. Last number measured actually an estimate 2 1 3 4 5
Sig Figs What is the smallest mark on the ruler that measures 142.15 cm? 142 cm? 140 cm? Does the zero mean anything? (Is it significant?) They needed a set of rules to decide which zeroes count.
Sig Figs. 405.0 g 4050 g 0.450 g 4050.05 g 0.0500060 g
Sig Figs Only measurements have sig figs. Counted numbers are exact – infinite sig figs A dozen is exactly 12
Problems 50 has only 1 significant figure if it really has two, how can I write it? Scientific notation 5.0 x 101 2 sig figs Scientific Notation shows ALL sig figs
Rounding rules Round 454.62 to four sig figs to three sig figs to two sig figs to one sig fig
Sig figs. How many sig figs in the following measurements? 458 g 4085 g 4850 g 0.0485 g 0.004085 g 40.004085 g
Density Density = mass D = m volume V Units: g/cm3 or g/mL but SI unit is kg/m3 derived unit Used to identify substances Varies with temperature As temp. increases density…
Density
Density Examples If a metal block has a mass of 65.0 grams and a volume of 22 cubic centimeters, what is the density of the block? D = m V D = 65.0 g = 3.0 g/cm3 22 cm3
Density Examples Aluminum has a density of 2.7 g/cm3. What volume of aluminum has a mass of 60 grams? D = M V 20 cm3
Unit Conversions
Conversion factors Given information in one unit need to find the equivalent in another unit “A ratio of equivalent measurements.” Start with two things that are the same. 1 m = 100 cm Can divide by each side to come up with two ways of writing the number 1.
Conversion factors =
Conversion factors 1 1 m = 100 cm
Conversion factors 1 1 m = 100 cm
Conversion factors 1 1 m = 100 cm 100 cm = 1 m 1
Conversion Factors Unique way of writing the number 1. Does NOT change the VALUE, it changes the UNITS.
Write the conversion factors for the following kilograms to grams 1 L = 1 dm3 = 1000mL = 1000 cm3
More Unit Conversions More Involved
Derived Unit Conversions 54.3 cm3 = ______ m3
Derived Unit Conversions 625 g/mL = ______ kg/m3
Where do these measurements come from? Recording Measurements
Making Good Measurements We can do 2 things: Repeat measurement many times - reliable measurements get the same number over and over - this is PRECISE
Making Good Measurements 2. Test our measurement against a “standard”, or accepted value - measurement close to accepted value is ACCURATE
Measurements are Uncertain Measuring instruments are never perfect Skill of measurer Measuring conditions Measuring always involves estimation Flickering # on balance Between marks on instrument
Estimating Measurements
Error Probably not EXACTLY 6.35 cm Within .01 cm of actual value. 6.35 cm ± .01 cm 6.34 cm to 6.36 cm
Calculating Percent Error Compares your measurement to accepted value Negative if measurement is small Positive if measurement is big
Calculating Percent Error What is the % error for a mass measurement of 17.7g, given that the correct value is 21.2g?
Direct Proportions Two quantities are directly proportional if dividing one by the other gives a constant y x “y is proportional to x” General Equation y = k x
Direct Proportions Solve for y: y = k x Look familiar? Eqn for a straight line: y = mx + b Slope is the constant
Direct Proportion
Inverse Proportions Two quantities are inversely proportional if their product is a constant “y is proportional to 1 divided by x” General eqation: xy = k Example: speed and travel time
Inverse Proportion Graph is called “hyperbola”
Calculations Convert 3.23 x 104 kg to g. Give answer with correct sig. figs.
Calculations What is the mass of an object with a density of 25.98 g/mL and a volume of 4.2 mL? What is the density of a 430 g object that takes up 25.5 cm3?