Measuring Length by the HandfulThe 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 10.16 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, simpleEx. 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 0.001

PREFIX Symbol multiplier

2. Derived units – combining different units using math

Speed = distance unittime 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 7.778 s