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 Originally defined as the 1/10,000,000 of the distance between the North Pole and the Equator. –Circa 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/ 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.

Which group of data has better precision? Trial Measurements Group 1Group Average12

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

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?