Transcript phys141 lecture - Texas Southern University

Phys141 Principles of Physical Science
Chapter 1
Measurement
Instructor: Li Ma
Office: NBC 126
Phone: (713) 313-7028
Email: [email protected]
Webpage: http://itscience.tsu.edu/ma
Department of Computer Science & Physics
Texas Southern University, Houston
Sept. 8, 2004
Measurement
• Measurements in our daily life:
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length/height
weight/mass
time
temperature
pressure
Others
• First step to understand our physical environment
• Describe the nature
• Use our senses to make measurements
The Senses
• 5 senses make it possible to know the environment:
 sight, hearing, touch, taste, smell: provide information
• Limitations of sense:
 for example, telescope to distinguish the stars,
microscope for the cell
 may also provide false information
 can be reduced or eliminated by using measuring
instruments
• Instruments have their limitations: accuracy limits
• Scientific method will tell us how to deal with them
Systems of Units
• Units
− Express measurements
− Describe things in a concrete way – that is, numerically
• Standard unit
− fixed and reproducible value for the purpose of taking
accurate measurements
• Systems of units
− Metric system
− British system
Systems of Units (cont)
• Fundamental physical quantities
− length, mass, time, etc.
• Length
− Description of space: location/size
− Measurement of space in any direction: length
− “meter” in metric system
− “foot” in British system
Systems of Units (cont)
• Mass
− The amount of matter an object contains
− Can be defined in terms of force and acceleration, and
gravity
− “kilogram” in metric system
− Mass is fundamental quantity, not weight:
Mass doesn’t change when measured in
different place
Systems of Units (cont)
• Time
− Duration, period, interval
− The continuous, forward flowing of events
− Only one direction – forward
− The 4th dimension of the space
− “second” in both metric system and British system
More on Metric System
• mks system
• International Standard System (SI): 7 base units
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Meter (m)
Kilogram (kg)
Second (s)
Ampere (A): measure the flow of electric charge
Kelvin (K): measure the temperature
Mole (mol): measure the amount of a substance
Candela (cd): measure luminous intensity
More on Metric System (cont)
• Metric prefixes
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mega- (M): 1,000,000 (million)
kilo- (k): 1,000 (thousand)
centi- (c): 0.01 (hundredth)
milli- (m): 0.001 (thousandth)
• cgs system: centimeter, gram, second
• Decimal (base-10) system
− Simpler when converting from one unit to another
Derived Units
Derived Quantity
Unit
Area (length2)
m2, cm2
Volume (length3)
m3, cm3
Speed (length/time)
m/s, cm/s
Density (mass/volume)
kg/ m3, g/cm3
Conversion Factors
• Conversion Factor
− Relate one unit to another: 1 in. = 2.54 cm
− Ration from an equivalence statement
• Steps for converting:
• Step 1: choose a conversion factor
• Step 2: cancel the unwanted unit
• Step 3: check the remained unit
Significant Figures
• Rules for number of significant figures to keep in
operations
− In multiplying and dividing, the least number of significant
figures
− In adding and subtracting, the least number of decimal
places
• Rules for rounding
− If the first digit to be dropped is less than 5, leave the
preceding digit as it
− If the first digit to be dropped is 5 or greater, increase the
preceding digit by 1
Scientific Notation
• Power of 10
− 1000 = 1x103 = 1E3
− 26400000 = 2.64x107 = 2.64E7
− 0.00000264 = 2.64x10-6 = 2.64E-6
• Rules for using this notation
− The exponent, or power of 10, is increased by 1 for every
place the decimal point is shifted to the left
− The exponent, or power of 10, is decreased by 1 for every
place the decimal point is shifted to the right