physics and measurement
DESCRIPTION
Physics and Measurement. What is physics? Why do we study it?. Model Theory Observation Law Empirical Law. Scientific notation helps!!!. - PowerPoint PPT PresentationTRANSCRIPT
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Physics and Measurement
• Model• Theory• Observation• Law• Empirical Law
What is physics?Why do we study it?
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Significant Digits• 23.21 m• 0.062 m• 8200 m
26.2x10 m
Scientific notation helps!!!
2Area 0.062 m 23.21 m 1.43902 m
• The final result of multiplication or division can have only as many significant digits as the component factor with the least number of significant figures
• The final result for addition or subtraction can have no more decimal places than the term with the least number of decimal places
2Area 1.4 m
Perimeter = 46.54 m
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Base Units
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Redefining the meter:
In 1792 the unit of length, the meter, was defined as one-millionth the distance from the north pole to the equator.
Later, the meter was defined as the distance between two finely engraved lines near the ends of a standard platinum-iridium bar, the standard meter bar. This bar is placed in the International Bureau of Weights and Measures near Paris, France.
In 1960, the meter was defined to be 1 650 763.73 wavelengths of a particular orange-red light emitted by krypton-86 in a discharge tube that can be set anywhere in the world.
In 1983, the meter was defined as the length of the path traveled by light in a vacuum during the time interval of 1/299 792 458 of a second. The speed of light is then exactly 299 792 458 m/s.
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Typical Lengths
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Typical Masses
What is Density?Vm
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Typical Times
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Three unit systems
Physical Quantity
Dimensional Symbol
UnitSystem
SI MKS SI CGS USCustomary
Length [L] m cm ft
Mass [M] kg g
Time [T] s s s
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Three unit systems
Physical Quantity
Dimensional Symbol
UnitSystem
SI MKS SI CGS USCustomary
Length [L] m cm ft
Mass [M] kg g slug
Time [T] s s s
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Unit conversion
A BA 1B
C 1 C
1 kg 2.2 lb 1 kg 12.2 lb
1 kg200 lb 90.9 kg2.2 lb
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Metric Prefixes
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Dimensional (unit) Analysis
21x at2
22
1 L[L] [ ][T ] [L]2 T
If your units do not work out, your answer cannot be correct!
Sometimes you can figure out the correct equation merely by making the units work!
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Estimating• Often we are looking for order of magnitude
numbers. • Is the number 1, 10, 100, 1000, 10000?• Make assumptions. We will have some
standard assumptions:– Surfaces are frictionless (at first)– Strings have no mass– Objects are all treated as if their mass is at a point in
space– Pulley wheels have no mass– Forces from springs are linear with displacement
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Example• Enrico Fermi • Nothing to do with Physics• Shows the power of order of magnitude estimates
• How many piano tuners are in San Francisco?(800,000 people in San Francisco)
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Lab Data Recording
And Calculating
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Uncertainty
• Precision = Repeatability• Accuracy = Correctness
measurement = best measured value uncertainty
uncertaintypercent uncertainty = 100%best measured value
uncertaintyfractional uncertainty = best measured value
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Propagating Uncertainty• Addition/Subtraction: Add the uncertainty in the
individual terms
• Multiplication/Division:– Add the fractional uncertainties of the factors
– Use extreme values of factors and subtract
If C=A+B, Then C= A+ B
C A BIf C=AB, Then = + C A B
If C=AB, Then C= A+ A B+ B A - B
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What does it mean to agree?
5.0
0.5
0.5
4.8
0.25
0.25
Measurement A Measurement B
Do Measurements A and B agree?
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What does it mean to agree?
5.0
0.5
0.5
4.2
0.25
0.25
Measurement A Measurement B
Do Measurements A and B agree?
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What does it mean to agree?
5.0
0.5
0.5
4.6
0.25
0.25
Measurement A Measurement B
Do Measurements A and B agree?
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What does it mean to agree?
5.0
0.5
0.5
4.4
0.25
0.25
Measurement A Measurement B
Do Measurements A and B agree?