Transcript Description and Measurement

Description and Measurement
Ms. Pollock
8th 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