Distinguish between accuracy and precision.

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Section 1.2 1
In this section you will

• Distinguish between accuracy and precision.
• Determine the precision of measured quantities.

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Section 1.2 2
What is a Measurement?

• A measurement is a comparison between an unknown
  quantity and a standard.
• Measurements quantify observations.
• Careful measurements enable you to derive the
  relationship between any two quantities.

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Section 1.2 3
Comparing Results

• When a measurement is made, the results are often
  reported with uncertainty.
• Therefore, before fully accepting new data, other
  scientists examine the experiment, looking for
  possible sources of errors, and try to reproduce
  the results.
• A new measurement that is within the margin of
  uncertainty confirms the old measurement.

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Section 1.2 4
Precision Versus Accuracy
Click image to view the movie.
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Section 1.2 5
Techniques of Good Measurement

• To assure precision and accuracy, instruments
  used to make measurements need to be used
  correctly.
• This is important because one common source of
  error comes from the angle at which an instrument
  is read.

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Section 1.2 5
Techniques of Good Measurement

• To understand this fact better, observe the
  animation on the right carefully.

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Section 1.2 6
Techniques of Good Measurement

• Scales should be read with ones eye straight in
  front of the measure.
• If the scale is read from an angle, as shown in
  figure (b), you will get a different, and less
  accurate, value.

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Section 1.2 6
Techniques of Good Measurement

• The difference in the readings is caused by
  parallax, which is the apparent shift in the
  position of an object when it is viewed from
  different angles.

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Section 1.2 7
Question 1

• Ronald, Kevin, and Paul perform an experiment to
  determine the value of acceleration due to
  gravity on Earth (which most scientists agree is
  about 980 cm/s2). The following results were
  obtained Ronald 961 12 cm/s2, Kevin 953
  8 cm/s2, and Paul 942 4 cm/s2. Determine who
  has the most accurate and precise value.

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Section 1.2 7
Question 1
A. Kevin got the most precise and accurate
value. B. Ronalds value is the most accurate,
while Kevins value is the most precise.
C. Ronalds value is the most accurate, while
Pauls value is the most precise. D. Pauls value
is the most accurate, while Ronalds value is the
most precise.
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Section 1.2 8
Answer 1
Reason Ronalds answer is closest to 980 cm/s2.
Hence, Ronalds result is the most accurate.
However, Pauls error is only 4 cm/s2. Hence,
Pauls result is the most precise.
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Section 1.2 9
Question 2

• What is the precision of an instrument?

A. the smallest divisions marked on the
instrument B. the least count written on the
instrument C. one-half the least count written on
the instrument D. one-half the smallest division
written on the instrument
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Section 1.2 10
Answer 2
Reason Precision depends on the instrument and
the technique used to make the measurement.
Generally, the device with the finest division on
its scale produces the most precise measurement.
The precision of a measurement is one-half of the
smallest division of the instrument.
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Section 1.2 11
Question 3

• A 100-cm long rope was measured with three
  different measuring tapes. The answer obtained
  with the three measuring tapes were 1st
  measuring tape 99 0.5 cm, 2nd measuring tape
  98 0.25 cm, and 3rd measuring tape 99 1
  cm. Which measuring tape is the most precise?

A. 1st measuring tape B. 2nd measuring
tape C. 3rd measuring tape D. Both measuring
tapes 1 and 3
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Section 1.2 12
Answer 3
Reason Precision depends on the instrument. The
2nd measuring tape has an error of only 0.25 cm
and is therefore the most precise.
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