Measurement

1
Measurement

• Predictions, Ch. 1, Activity 2

2
Measurements

• What form of measurement existed 2,000 years ago?
• Parts of the body were originally used
• Palms
• Hands (in use today)
• Digits
• Feet (in use today)
• Cubit (still referenced today)

3
Measurements

• Circumference of the Earth was determined by
  Erathosthenes (276 195 B.C.) by comparing the
  noontime shadows in two different cities and
  determining the differing angle of the sun at
  each location
• He assumed a spherical Earth

4
SI System

• Sytems International
• Regulated by the International Bureau of Weights
  and Measures in France.
• NIST (National Institute of Science and
  Technology in Maryland).
• SI maintains the standards for
• Length (meter)
• Time (second)
• Mass (kilogram)

5
Standard Units of Measurement

• Meter (m)
• Circa 1790 - Originally defined as the
  1/10,000,000 of the distance between the North
  Pole and the Equator.
• Circa 1890 - defined as the distance between two
  lines on a platinum-iridium bar.
• In 1983 it was defined as the distance that light
  travels in a vacuum in 1/299792458 s.

6
Measurement Error

• Random error
• Error that occurs due to the inherent uncertainty
  in any measurement tool.
• Error that is the result of the natural
  variability in any measurement.
• Systematic error
• Error that occurs due to improperly reading or
  recording a measurement.

7
Precision

• Precision is a measure of the repeatability of a
  measurement. The smaller the variation in
  experimental results, the better the
  repeatability.
• Precision can be improved by instruments that
  have high resolution or finer measurements.
• e.g. A ruler with millimeter (mm) divisions has
  higher resolution than one with only centimeter
  (cm) divisions.

8
Which group of data has better precision?
Trial Measurements Measurements
Trial Group 1 Group 2
1 10 10
2 15 11
3 5 14
4 13 13
5 17 12
Average 12 12
9
Accuracy

• How close are your measurements to a given
  standard?
• Accuracy is a measure of the closeness of a body
  of experimental data to a given known value.
• In the previous table, the data would be
  considered inaccurate if the true value was 15,
  whereas it would be considered accurate if the
  standard value was 12.

10
Estimation

• When a precise measurement is not necessary use
  an estimate.
• An estimate relies on past experiences and good
  judgment.
• A college football player has a mass of 100 kg.
• Since 100 kg is about 220 lbs, this appears to be
  a good estimate.
• A high school basketball player is 4 m tall.
• 4 m is about 14 feet! Not a good estimate.

11
Estimation

• Your teacher works 1440 minutes every day.
• That is every minute in 24 hours!
• A poodle has a mass of 60kg
• Not a toy poodle. Maybe a standard poodle.
• Your classroom has a volume of 150m3.
• Not too far off
• The distance across the school grounds is 1km.
• Again, close.

12
Precise Measurements

• How would you record this measurement using a
  centimeter ruler?
• 5.15 centimeters.
• Measure to the smallest division 0.5.

13
Precision What do you think?

• An Olympic swimming pool is 50 m long. How
  precise do you think that is?
• An oil tanker holds 5x106 barrels of oil. How
  precise is this?
• A 2 x 4 piece of lumber is a standard size.
  How precise do you think this measurement is?

14
Measurement tools

• When would you use each of these?
• Meterstick
• Ruler
• Measuring tape
• Vernier caliper
• Micrometer caliper

15
Measurement

• What does it mean to calibrate?