Welcome to Physics

1
Welcome to Physics

• 2009-2010

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Measurement

• Who is the biggest kid in the school?
• Height, weight, both?
• Have to be specific with measurements, units, sig
  figs.

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Uncertainty

• No measurement is absolutely precise
• Usually within /- 1 of the smallest division on
  the measuring device.
• 22.1cm /- .1 (22.2-22.0)
• Therefore no calculations made with a measurement
  are absolutely precise.
• Calculations must be rounded off using only the
  Significant Figures

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Significant Figures

• The number of reliably known digits in a number.
• The length of a book can be measured to about
  22.1 cm
• 22.1 has 3 SF
• Find the area of the front cover of the book if
  its dimensions are 22.1x28 cm
• 618.8 cm2 is calculator answer.
• The number of SFs in a final result should be
  the same as the least significant input value.
• In this case the answer is 620 cm2
• If the zero were significant, how could we show it

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Rules of SF

• Multiplication and division-answer has the same
  number of digits as the least significant number
  in the problem
• 24.68 x 8.6 212.25 What is this with SF?
• 210
• Addition or subtraction answer should not have
  digits beyond the last decimal point common to
  all numbers being added or subtracted.

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Rules of SF

• 5.84 122.1 54.67 182.61
• What is this with SF?
• 182.6

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Correlation of units

• Find the volume of a book with dimensions
• 12 in x 27cm x 36mm
• What do you need to do first?

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Convert to SI units

• 12in(1m/39.37in) 0.30480061m
• 27cm(1m/100cm) 0.27m
• 36 mm(1m/1000mm) 0.036m
• Should we put these answers in SF before
  completing the calculations?
• No! Go ahead and solve.
• 0.002962662 becomes
• 0.0030 m3
• Why dont we need a bar over this zero?
• First 2 zeros are place holders, the ending 30
  are significant

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Major SI units

• Parameter measured
• Length
• Time
• Mass
• Electric current
• Temperature
• Amount of substance
• Luminous intensity

• SI unit
• Meter (m)
• Second (s)
• Kilogram (kg)
• Ampere (A)
• Kelvin (K)
• Mole (mol)
• Candela (cd)

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Why SI is important

• Sharing info
• Later derived units depend upon using standard
  base units.
• Ex Unit for force is a ___________
• Newton is a kgm/s2 If you are asked to find the
  mass of an object that weighs 50N, you have to
  use the correct units for acceleration to get a
  valid answer

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Order of Magnitude

• All calculated answers should be quick checked
  for order of magnitude.
• Does my answer make sense?
• Runners speed is 500 m/s?
• 3.8x103 x 2.95x108 1.121x102 ?
• Checking to be sure your calculation is on the
  correct order of magnitude will help recognize
  math errors.

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Just for fun

• In 1959, members of an MIT fraternity measured
  the nearby Massachusetts Avenue Bridge by rolling
  fellow student, Oliver Smoot, end to end across
  it and painting a mark after each length. The
  paint marks have been kept fresh ever since, and
  you can still read 364.4 SMOOTS at the MIT end
  of the bridge. Assume a reasonable length for
  Oliver, and estimate the length of the bridge in
  meters.
• If the actual length is 647.9 m, how tall is
  Oliver in m? in ft?
• 1.778 m, 5.721 ft or about 5 9

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Homework

• Page 16 Questions 4, 7, 8
• Problems 2,3,4,12,17,21