Physics 10 Lab 1: Introduction to Measurement
Units and Measurement
One of the most important steps in applying the scientific method is experiment: testing the prediction
of a hypothesis. Typically we measure simple quantities of only three types: mass, length, and time.
Occasionally we include temperature, electrical charge or light intensity. It is amazing, but just about
everything we know about the universe comes from measuring these six quantities. Most of our
knowledge comes from measurements of mass, length, and time alone. We will use the standard used
by the international scientific community for measuring these quantities: the SI metric system. A
measurement without a unit is meaningless! For more information, check out these websites:
SI Base Units: http://physics.nist.gov/cuu/Units/units.html
Metric prefixes: http://physics.nist.gov/cuu/Units/prefixes.html
Commonly used metric prefixes
Often it is necessary to convert from one unit to another. To do so, you need only multiply the given
quantity by a conversion factor which is a ratio equal to 1, derived from definitions. For example, there
are 100 centimeters in a meter. You can make a conversion factor out of that definition so that if you
need to convert 89cm to meters, simply multiply by the conversion factor, which is equal to 1:
100cm=1m →
89cm×
1m
=1
100cm
1m
=.89m
100cm
You can make conversion factors out of the definitions listed in the tables above and in your book.
Precision & Accuracy
By their nature, measurements can never be done perfectly. Part of the error in making measurements
may be due to the skill of the person making the measurement, but even the most skillful among us
cannot make the perfect measurement. Basically this is because no matter how small we make the
divisions on our ruler (using distance as an example) we can never be sure that the thing we are
measuring lines up perfectly with one of the marks. Therefore the judgment of the person doing the
measurement plays a significant role in the accuracy and precision of the
measurement.
Accuracy: Accuracy describes the nearness of a measurement to the standard or true
value, i.e., a highly accurate measuring device will provide measurements very close
to the standard, true or known values. Example: in target shooting a high score
indicates the nearness to the bull's eye and is a measure of the shooter's accuracy.
Precision: Precision is the degree to which several measurements provide answers
very close to each other. It is an indicator of the scatter in the data. The lesser the
scatter, the higher the precision.
Ideally, we want to make measurements that are both accurate AND precise.
However, we can never make a perfect measurement. The best we can do is to
come as close as possible within the limitations of the measuring instruments.
Uncertainty
Since we can never make a
perfect measurement, every
measurement is approximate. Therefore it is important to always report the amount of confidence we
have in our measurements, what we call experimental uncertainty. For example, you may estimate the
length of the lab bench to be “5 meters give or take a meter”. The “give or take” part is an expression of
your confidence in your estimate. In scientific measurements we say “plus or minus” but it means the
same as “give or take.” We write that our measurement of the length, represented by “L” is :
L = 5m + /-1m
If you are using a scale such as a ruler to measure the length of an object, then
your uncertainty is usually estimated to be one tenth the smallest division. For
example, this bug has a length between 1.54 and 1.56 in or
L = 1.55in + /-0.01in .
The 1.55in is the average measure and the 0.01in is the uncertainty.
Error
An experimental error is not a mistake! It is the difference between a measurement and an accepted
value of something. For example, if you determine from an experiment that the acceleration due to
gravity is 10 m/s2 then the ‘error’ is the difference between that value and the accepted value of 9.8m/s2,
or 0.2m/s2. The error can also be expressed as a percent:
% error =
10 − 9.8
×100% = 2%
9.8
Physics 10
Print Name: ________________________________
Lab 1: Measurement Worksheet
Lab Partner: ________________________________
Equipment: ½, 1 and 2 meter rulers, digital caliper, various small metal objects
Part I: Units, Metric Prefixes & Unit Conversion Show calculations. NEATNESS COUNTS!!
1. Convert 32 kilometers to nanometers.
15
2. If 1 light year = 9.46 x 10 m and 1 mile = 1.6 km, how many miles are in a light year?
3. Explore the Universe in powers of 10! http://micro.magnet.fsu.edu/optics/tutorials/java/powersof10/
Step through the animation in manual mode. What is the power of 10 for each of the following?
Milky Way Galaxy: ___________
Stars in the Milky Way Galaxy: ________________
Solar System: ______________
Earth and the Orbit of the Moon: _______________
Southwest Tallahassee: __________ Oak Tree Branch: _________Cells on a Leaf: __________
DNA Strand: __________ Nucleus of a Carbon atom: __________ Quark: ___________
By what power of 10 is the Milky Way Galaxy larger than the Nucleus of a Carbon Atom? Show your
calculation. Box your final answer.
Part 2. Precision and Accuracy Go to this website and shoot the bulls’s eye and test your
understanding of accuracy and precision: http://www.utas.edu.au/sciencelinks/exdesign/EE3B.HTM
Then, identify the following as either being Precise and Accurate, both or neither, by circling the words.
Precise
Accurate
Precise
Accurate
Precise
Accurate
Precise
Accurate
Part 3. Measuring Length & Uncertainty
Make a thumb ruler by marking the size of your thumb starting on the left line and across the strip
below. You will use the thumb ruler to measure the length of the computer screen but first GUESS the
length in ‘thumbs’ by ‘eye balling’ it. Be sure to include +/- uncertainty in your guess and
measurement.
Guesstimate: _____________________
Measurement: ___________________________
How close was your guess? How many thumbs off were you? ____________________________
Measuring with a metric ruler. When using a metric ruler, your uncertainty – to the nearest one tenth
of the space between the smallest scale divisions. Here are two metric rulers. Write the measurement in
standard form with +/- uncertainty. Don’t forget the units!
Ruler on Left: ________________________
Ruler on Right: _______________________
Now measure the computer screen with a metric ruler. Again, first GUESS the length in cm by ‘eye
balling’ it. Be sure to include +/- uncertainty in your guess and measurement.
Guesstimate: _____________________
Measurement: ___________________________
How close was your guess? By what percent was your guess off? __________________________
Measure the length of the metal object with a metric ruler. Write the measurement in standard form
with +/- uncertainty. Don’t forget the units!
Metric Ruler Length: __________________________
Measure the length of the metal object with a digital meter. In general, when using a digital meter,
the uncertainty is ½ the digit not shown. What is that for your digital meter?
Digital Meter Uncertainty: __________________________
Write the measurement in standard form with +/- uncertainty. Don’t forget the units!
Digital Meter Length: _________________________
Part 4: Measurement and Perception
Our eyes and minds can deceive us and produce errors in our measurements and perceptions! Using
observation only, answer the questions in the column on the left. Then measure the objects to the
nearest tenth of a millimeter. Then answer the questions in column on the right. Can you trust your own
eyes? Be honest!!