How Science works : Taking measurements

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Teachers Notes This sequence of slides is
designed to introduce, and explain the taking of
measurements, including the meaning of variation,
range, mean (average) and the difference between
accuracy precision, as explained on page 362 in
New Physics for You, 2006 edition. On each slide
the key points are revealed step by step, at the
click of your mouse (or the press of a key such
as the space-bar). Before making the next
mouse-click you can ask questions of the class or
make statements about what is about to be
revealed. This should help students to become
clearer about the ideas involved. Naturally it
pays to have quick practice-run first. To start
the slide-show, press function-key F5 (or
right-click-gtFull Screen)(to return to normal
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How Science works
Takingmeasurements
New Physics for You, page 362
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Learning Objectives You should learn

• About taking measurements,
• The meaning of variation, range and mean
  (average),
• The meaning of accuracy and precision.

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Taking measurements
When you take measurements there may be some
variation in the readings.
For example
If you time the fall of a paper parachute over a
fixed distance, the times may vary slightly.
10.1 s, 10.2 s, 9.9 s, 10.0 s, 10.3 s
Lets look at these results more closely.
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Taking measurements
The results were 10.1 s, 10.2 s, 9.9 s,
10.0 s, 10.3 s What is the Range of these
results?
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Taking measurements Range
The results were 10.1 s, 10.2 s, 9.9 s,
10.0 s, 10.3 s
Find the minimum value
and the maximum value
Range
from min to max
9.9 to 10.3
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Taking measurements Mean
The results were 10.1 s, 10.2 s, 9.9 s,
10.0 s, 10.3 s
What is the mean (or average) of these results?
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Taking measurements Mean
The results were 10.1 s, 10.2 s, 9.9 s,
10.0 s, 10.3 s
Add up the 5 numbers 10.110.29.910.010.3
50.5 There are 5 items, so divide by 5 Mean (or
average)
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Taking measurements Mean
The results were 10.1 s, 10.2 s, 9.9 s,
10.0 s, 10.3 s
Why is it a good idea to calculate the mean of
your results? Because it improves the reliability
of your results. Your results will be more
reliable.
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Accuracy
and
Precision
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Definitions
Accuracy and Precision sound the same thing
is there a difference??
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Definitions Accuracy
In your experiments, you need to consider the
accuracy of your measuring instrument.
For example
An expensive thermometer is likely to be more
accurate than a cheap one.
It will give a result nearer to the true value.
It is also likely to be more sensitive.So it
will respond to smaller changes in temperature.
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Definitions Precision
As well as accuracy, precision is also important.
Precision is connected to the smallest scale
division on the measuring instrument that you
are using.
For example
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Definitions Precision
For example, using a ruler
A ruler with a millimetre scale
will give greater precision than a ruler with a
centimetre scale.
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Definitions Precision
A precise instrument also gives a consistent
reading when it is used repeatedly for the same
measurements.
For example
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Definitions Precision
For example, 2 balances
A beaker is weighed on A, 3 times The readings
are 73 g, 77 g, 71 g
So the Range is
It is then weighed on B, 3 times The readings
are 75 g, 73 g, 74 g So the Range is
Balance B has better precision.
Its readings are grouped closer together.
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Accuracy compared with Precision
Suppose you are measuring the length of a wooden
bar
The length has a true value
And we can take measurements of the length, like
this
Lets look at 3 cases
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Accuracy compared with Precision
Precise (grouped) but not accurate.
Accurate (the mean) but not precise.
Accurate and Precise.
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Learning Outcomes You should now understand

• The meaning of variation and range,
• How to calculate the mean (or average),
• and why this improves the reliability of your
  results,
• The difference between accuracy and precision.

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• For more details, see
• New Physics for You, page 362
• For more free PowerPoints, visit
• the web-site at www.physics4u.co.uk

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