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Physics and Measurement

Physics and Measurement. What is physics? Why do we study it?. Model Theory Observation Law Empirical Law. Scientific notation helps!!!.

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Physics and Measurement

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  1. Physics and Measurement What is physics? Why do we study it? • Model • Theory • Observation • Law • Empirical Law

  2. Scientific notation helps!!! • 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 Perimeter = 46.54 m Significant Digits • 23.21 m • 0.062 m • 8200 m

  3. Base Units

  4. 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.

  5. Typical Lengths

  6. Typical Masses What is Density?

  7. Typical Times

  8. Three unit systems

  9. Three unit systems

  10. Unit conversion

  11. Metric Prefixes

  12. Dimensional (unit) Analysis 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!

  13. 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

  14. 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)

  15. Lab Data Recording And Calculating

  16. Uncertainty • Precision = Repeatability • Accuracy = Correctness

  17. 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

  18. 0.5 5.0 0.5 0.25 4.8 0.25 What does it mean to agree? Measurement A Measurement B Do Measurements A and B agree?

  19. 0.5 5.0 0.5 0.25 4.2 0.25 What does it mean to agree? Measurement A Measurement B Do Measurements A and B agree?

  20. 0.5 5.0 0.5 0.25 4.6 0.25 What does it mean to agree? Measurement A Measurement B Do Measurements A and B agree?

  21. 0.5 5.0 0.5 0.25 4.4 0.25 What does it mean to agree? Measurement A Measurement B Do Measurements A and B agree?

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