Description and Measurement

Ms. Pollock8th Grade Physical Science

2009 - 2010

Measurement

Way to describe world using numbers

how much, how long, how far

more reliable than opinion

describes events– Olympics

Estimation

Rough measurement using something

familiar to guess size of new object

based on previous experience– chefs– firefighters

Estimation

Use “about” check for reasonable

answers– doorknobs about 1 m

from floor– sack of flour about 2

kg– walk about 5 km/h

Precision and Accuracy

Precision: how close measurements are to each other

same every time also number of decimal

places possible with particular tool

degrees of precision

Precision and Accuracy

Accuracy: comparison of measurement to real, actual, or accepted value

How close you are to the desired value

Precision and Accuracy

Important to medical procedures– Stereotactic

Radiotherapy (SRT)– treatment of brain

cancer without damaging healthy cells

Rounding a Measurement Some instruments not capable of great precision rounding rules important for estimation

– digit to right of digit being rounded is 0, 1, 2, 3, or 4: number remains same

– digit to right of digit being rounded is 5, 6, 7, 8, or 9: number increases by one

– digits to right of digit being rounded are right of decimal: delete them

– digits to left of digit being rounded are left of decimal: change to zeros

Rounding a Measurement

Length of sidewalk 135.841 m rounded to tenths place

– 135.8 m– digit to right of tenths place (number 8) was 4

rounded to ones place– 136 m– digit to right of ones place (number 5) was 8

Precision and Number of Digits

Rounding necessary when numbers do not divide evenly

digits that reflect precision significant– digits other than zero– final zeros after decimal point (6.545 600 g)– zeros between other digits (507.0301 g)– initial zeros NOT significant (0.000 2030 g)– zeros in whole number possibly significant (1650)– numbers counted, rather than measured

Applying Math: Rounding

The mass of one object is 6.941 g. The mass of a second object is 20.180 g. You need to know these values only to the nearest whole number to solve a problem. What are the rounded values?

6.941 g (number to right of ones place = 9)– 7 (rounded up)

20.180 g (number to right of ones place = 1)– 20 (remained same)

Following the Rules

Rules for determining significant figures in calculations– multiplication and division

• determined by number with fewer digits• 6.14 X 5.6 = 34.384• 3 2 2

– addition and subtraction• least precise place value• 6.14 + 5.6 = 11.74• 100s 10s 10s