Measuring Length by the Handful The English units that we use in the United States developed over a long period of time. For example, the hand was devised in ancient times as a unit of length. It was defined as the length of a person’s hand from the little finger to the thumb. Today the height of horses is still measured in hands, but the definition of a hand is standardized at 4 inches or centimeters. 1.Why did the hand produce unreliable measurements before it was standardized? 2. Measure the height of your desk in hands. Compare your results with other classmates. How do the results vary? Starter 9/14

MEASUREMENT Measurements must have a number and unit to make sense  5 cm. Scientists use the SI system (SI = METRIC)

UNITS Two types : base units and derived units 1. Base unit – basic, simple Ex. meter, liter, gram, seconds ** add prefixes to make larger or smaller  millisecond, kilogram

Base Units : Length – meters (m) Mass – gram (g) Time – seconds (s) Volume – liter (L)

Kilo k 1,000 hecto h 100 deca da 10 base: 1 deci d 0.1 centi c 0.01 milli m PREFIX Symbolmultiplier

2. Derived units – combining different units using math  Speed = distance unit time unit meters per second or (m/s)

Precision Precision – an EXACT measurement –Ex. Digital clocks tell the exact time –Ex. You sit in only one specific seat at the Phillies game.

Accuracy Accuracy- measurement that is CLOSE to the actual number. –It’s consistent, but not exact. –Ex. Same ball park, different seat each game –Ex. Setting a clock – keeps time but may not be the precise time

Starter 9/15 Look around your seat and find 2 items that are 5 mm in length Find 2 items that are 3 cm in length.

Significant Figures Significant Figures – all the digits of a measurement that are KNOWN. –More digits = more precise –Track time = 7.8 s or s