Measurement Predictions, Ch. 1, Activity 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)
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
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)
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.
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.
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.
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.
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.
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 150m^3. • Not too far off • The distance across the school grounds is 1km. • Again, close.
Precise Measurements • How would you record this measurement using a centimeter ruler? • 5.15 centimeters. • Measure to the smallest division + 0.5.
Precision: What do you think? • An Olympic swimming pool is 50 m long. How precise do you think that is? • An oil tanker holds 5x10^6 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?
Measurement tools • When would you use each of these? • Meterstick • Ruler • Measuring tape • Vernier caliper • Micrometer caliper
Measurement • What does it mean to calibrate?