STANDARDS OF MEASUREMENT 1 WHY DO WE NEED

STANDARDS OF MEASUREMENT 1

WHY DO WE NEED TO MEASURE? Measurements are needed to describe changes over time, to observe objects’ size and motion, …; in other words, to understand our Universe 2

“A science is as mature as its measurement tools” Louis Pasteur, 1822 -1895 “To measure is to know” James Clerk Maxwell, 1831 -1879 3

I. UNITS OF MEASUREMENT Data / values are communicated in terms of a NUMBER, representing the magnitude, quantities a UNIT describing the type of data being recorded. CAN YOU NAME SOME UNITS and TELL WHAT THEY REPRESENT? 4

STANDARDS are exact quantities of those units that people agree to use for COMPARISON. Those standards allow for a high degree of ACCURACY. 5

ACCURACY vs. PRECISION ? ? 6

ACCURACY vs. PRECISION Accurate repeated measurements are close to the accepted values, BUT… PRECISE repeated measurements are close to each other, BUT… 7

ACCURACY vs. PRECISION LOW ACCURACY 8

HOW WOULD YOU REPRESENT? ACCURATE and PRECISE NOT ACCURATE, NOR PRECISE 9

HOW WOULD YOU REPRESENT? ACCURATE and PRECISE 10

HOW WOULD YOU REPRESENT? NEITHER ACCURATE, NOR PRECISE 11

Can you explain why the following units or systems of measurements are not valid Standards? • The length of your foot • The width of your hand • The length between your thumb and pinky • The cubit –used by the Egyptians- being the length b/t the middle finger and the elbow • The uncia -used by the Romans- being the width of your thumb… 12

Draw a dart board on a sheet of paper and set it on the ground. Have each member in a group of five drop a marker on the target from a height of one meter, aiming for the bull's eye. Each mark represents a measurement of some quantity, say, the length of a textbook. One answer sheet per group. 1. What does the bull's eye represent? 2. How many length measurements were made? 3. Were the measurements accurate? Why or why not? 4. Were the measurements precise? Why or why not? Now simulate a new set of measurements by dropping markers from a height of 2 meters. 5. Has the accuracy and/or precision of your measurements changed? If so, how. Explain. 13

Suppose your team is challenged to determine the mass of an object using a balance. Each member of the team measures the mass on the same balance. Here are you data: Team Mass in member Grams 1 39. 97 2 40. 06 3 39. 98 4 39. 97 5 40. 02 Your team decides to report your mean (average) mass, which is 40. 00 g. Your instructor then informs you, however, that the actual mass is 45. 00 g. 1. Who (or what) would your instructor blame for the fact that your reported mass was too low by 5/45 = 1/9 or about 11%? 2. What can you conclude about the precision of your balance? 3. What can you conclude about the accuracy of your balance? 14