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Lecture1sf - SI Base Units Length Mass Time Amount...

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SI Base Units Length Meter Mass Kilogram Time Second Amount Mole Temperature Kelvin Current Ampere Luminous intensity Candela What is the definition of a meter? What is the definition of a kilogram? What is the definition of a second? Meter: Original: 000 , 000 , 10 1 of distance between the north pole and the equator. Defined 1960: 1,650,763.73 wavelengths of a particular orange-red line in Krypton-86 Modern (since 1983): The distance traveled by light in 458 , 792 , 299 1 second. Second: The amount of time corresponding to 9,192,631,770 oscillations of a cesium atom. Kilogram: The mass of a particular platinum-iridium cylinder (international prototype kilogram.
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SI Prefixes 10 12 tera T 10 9 giga G 10 6 mega M 10 3 kilo k 10 0 base unit 10 -3 milli m 10 -6 micro μ 10 -9 nano n 10 -12 pico p Standard prefixes are used for every three powers of 10. Exception: 10 -2 centi c This is used mostly for centimeter (cm)--a common unit of length and cm 3 , a common unit of volume. Derived Units There are many “derived” units which are combinations of the base units. Area m 2 , cm 2 Volume m 3 , cm 3 Force N = Newton = kg 2 sec m Energy Joule = N m = kg 2 2 sec m Pressure P = Pascal = 2 m N = 2 sec m kg Density 3 m kg 3 cm g Liter : Commonly used measure of volume Liter = 1000 cm 3 or 1 cubic decimeter (dm 3 )
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What is the difference between mass and weight? Weight : Equivalent to force (unit is Newtons). A measure of gravitational attraction. Mass Rough definition: Amount of matter Better definition: A measure of a body’s inertia. Inertia is the resistance to change in motion (acceleration). Significant Figures Experimental measurements have an uncertainty . The uncertainty depends on the measuring instrument used. Book example: Measuring weight of grapefruits: Bathroom Scale Balance Grapefruit 1 1.5 lb 1.476 lb Grapefruit 2 1.5 lb 1.518 lb These have the same weight to two significant figures (both are 1.5 lb) They have different weights with four significant figures (measured by balance). But even the balance has an uncertainty, in this case at least ± .001 lb. The uncertain digit is the last significant figure. We have some knowledge (with uncertainty) of that last significant digit. We have no knowledge of any digit past the last uncertain digit.
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