Everything we experience comes to us through our five senses—sight,
hearing, touch, smell and taste.
While our senses are truly amazing, most of what goes on around us
occurs unnoticed. Since we can only see a small range of the
electromagnetic spectrum as visible light, we can be in the
vicinity of a radio-transmitting tower radiating 50,000watts of power
and be totally unaware of its presence. The fluttering of a hummingbird
wing and changes in mountain ranges are undetectable to the average
human. Extreme distances, both short and long, are equally elusive. We
can see the dot above an “i”, but cannot see a grain of pollen. At the
other outer limits of length, we can only imagine what a light year is.
Scientific research includes the study of subatomic particles as well as
the mind-boggling distances that exist between the earth and neighboring
stars and nebulae. This great breadth of investigation involves
extending our senses and developing new ways of "seeing".
Scientific instruments that enable us to overcome our sensory limitations
have been, and continue to be, essential to the progress of science. The
microscope and the telescope provide mankind with windows to two
previously unseen worlds. The stroboscope has enabled us to “freeze”
motion. X-rays have provided a non-invasive way of probing the body.
Radio telescopes enable us to extend our grasp to the far reaches of
space. Cloud and bubble chambers allow us to study events occurring on
the subatomic scale.
In a similar way, the following experiments will allow your students to
extend their senses and make measurements they never dreamed possible.
They will determine the size of a molecule, time events that occur in an
instant, and measure dimensions on an astronomical scale. In the
process, they will learn how scientists make observations and
measurements in the invisible world.
Measuring New Heights
Students are asked to indirectly measure the height of an object much
larger than their available measuring instrument.
Give the students instructions below, and turn them loose! Give them time
to plan in the classroom before going out as a group to make
measurements. Give as little advice as possible. Their methods (and the
results) may vary a lot, and that’s okay!
Get details on special pricing for quantities purchases
of meter sticks here:
The challenge: Determine the height of a tall object on the school
grounds such as a flagpole, a chimney, or other structure identified by
your teacher. You will only be allowed the use of a meter stick. You may
think that you don’t have the knowledge to make such a measurement with
so little equipment, but you would be wrong!
Before leaving the classroom, do some
brainstorming with members of your group. You will be surprised to learn
that you have the ability to measure such a tall object indirectly. Each
group will be asked to share not only their value for the height of the
object, but more importantly, their method. So be ready!
The measurement method known as triangulation can be used to
indirectly determine the heights of tall structures or the altitudes of
Demonstrate the calculations for the class before assigning each group a
height or altitude to measure. Trigonometry is involved, but students
really only need to do some basic algebra to grasp this concept. You can
use triangulation to find the height of buildings or as part of other
labs and activities like Bottle Rockets.
Trigonometry provides an easy way to determine the heights of structures
or even the altitude of a toy rocket. Trigonometry deals with ratios of
the lengths of pairs of the sides of a right triangle. You may have
heard of the sine, cosine, and tangent. Scary sounding? Perhaps, but
don’t worry, they’re all just ratios. To make things easier, we’ll only
consider the tangent.
The tangent of an angle (Θ, “theta”) is the ratio of the length of the
side opposite the angle to the length of the side adjacent to the angle.
In other words, it’s the ratio of side a to side b. This ratio increases
as the angle of inclination Θ increases. The tangent for angles between
0 and 90 degrees may found in a table or calculator.
Get details on measurement supplies here:
Suppose you fire a rocket into the air and
wish to know its altitude. If you know the distance from you to the
launch point (b) and the angle of inclination (Θ), you can find the
rocket’s altitude (a) because
tangent Θ = a/b
a = b tangent Θ
(Hint: You can look up the tangent of any angle from 0 to 90 in a table or
by using your scientific calculator.)
Voila! The tangent makes the indirect measurement of heights a snap.
To actually carry out a measurement of a
rocket’s altitude, you will need a protractor (an instrument used for
measuring angles), a string with a small weight on the end (also known
as a plum bob), a meter stick, and a tangent table or calculator.
After tying a weight to the end of a string, attach the string to the
center of the protractor (see fig. 2). This device will enable you to
determine the angle of inclination. Now all you need is the baseline b,
the distance between the launch pad and where you stand when you sight
on the rocket.
When the rocket reaches its maximum altitude, view the rocket along the
edge of the protractor. Have your lab partner observe the angle
indicated by the string. Because the protractor is inverted, this angle
must be subtracted from 900 to obtain the angle of inclination. To find
the altitude of the rocket, simply multiply the tangent of the angle of
incidence by the length of the baseline.
Students will use the concept of similar
triangles to indirectly measure the diameter of the moon.
This activity can be done in the classroom, if
the moon happens to be visible from your windows, or it can be done at
home by each student.
You may find this hard to believe, but you can
measure the diameter of the moon from the comfort of your home.
The equipment needed includes an index card, a pin, two strips of opaque
tape (masking or electrical tape works well), and a centimeter ruler.
Oh, and one other thing, you’ll need to know
that the moon is 3 x 105 km from earth.
When the moon is full, place the two strips of
tape 2cm apart on a windowpane facing the moon. After making a
pinhole in the index card, observe the moon through the pinhole and two
strips of tape. Back away from the window until the moon appears
to just fill the space between the two strips of tape. Measure the
distance from the card to the window. Using the proportionality of sides
that exists for similar triangles (see figure above), calculate the
diameter of the moon.
Measuring Molecular Monolayers
Students will use the volume of a large number
of items and the area covered by a single layer of those items to
indirectly find the diameter of a single item.
Get details on the new arbor Scientific Molecular Size
and Mass Kit here:
any lab using chemicals and glassware, this lab requires appropriate
safety measures, such as goggles. The final result for the height
of an oleic acid molecule might not be very accurate, but the exercise
is still worthwhile. When students are able to measure something
that they cannot see, they understand a bit more about how scientists
Suppose you wanted to find the diameter of a BB, but didn’t have an
instrument, such as a micrometer, suited for the job. What could you do?
One way to obtain the diameter of a single BB requires the use of many
BB’s. Begin by placing a large number of BB’s in a graduated cylinder.
Record the total volume of BB’s. (In carrying out this measurement, you
are making an assumption. Do you know what it is?)
Now spread the BB's out in a circular pattern on a table. This results in
a monolayer, a cylindrical volume whose depth is a single BB. Measure
the diameter of this circle. Because you may have difficulty making a
perfect circle, make this measurement a number of times and find the
average diameter. Divide the diameter by two, and use this radius to
find the area of the circle.
The diameter of one BB is the same as the height of the very flat cylinder
you just made. We can use the area of the circle and the total volume of
BB’s (measured earlier) to find the height. The volume of a cylinder is
the area of its base times the height.
V = A x h
h = V/A
Note: 1 mL = 1 cm3.
If you have a micrometer, use it to check your answer.
Believe it or not, you can estimate the size of a single molecule using a
similar approach. This time however, you will be dealing with a
monolayer of molecules rather than a monolayer of BBs. To perform the
experiment you’ll need a pizza pan, some chalk dust, an eyedropper, a
10-ml graduated cylinder, and oleic acid solution. The oleic acid
solution is prepared by adding 5-ml oleic acid to 995-ml ethanol.
After filling the pizza pan with water, spread chalk dust over the surface
of the water. Easy does it, for too much powder will hinder the spread
of the oleic acid. Using an eyedropper, carefully add just one drop of
the oleic acid solution to the center of the pan. The alcohol will
dissolve in the water, but the oleic acid will spread out to form a
nearly circular shape. As you did with the BBs, measure the diameter of
this rough circle a number of times and find the average. Then find the
area of the circle.
Remembering that you put a single drop of oleic acid solution on the
surface of the water, you will have to determine the volume of acid in a
single drop of solution. To do this, count the number of drops needed to
occupy 1-ml in the graduated cylinder. Do this several times and take an
average. The volume of a single drop is found by dividing 1-ml (=1 cm3)
by the average number of drops in a cm3. The actual volume of oleic acid
is only 0.005 of the volume of a drop (Why?). Multiple the volume of a
single drop by 0.005 to obtain the volume of oleic acid.
Just as with BBs, you can now find the size of a single molecule by
dividing this volume by the area of the circle.
What assumption are you making regarding the shape of an oleic acid
Measuring Short Time Intervals with a Stroboscope
Stroboscopes are instruments that allow the
viewing of repetitive motion in such a way as to make the moving object
appear stationary. Stroboscopes may also be used to measure short time
Stroboscopes may either be mechanical or
electronic. Mechanical, or hand-held, stroboscopes consist of a disk
with equally spaced slits around its circumference. The disk is spun
around a handle while the viewer looks at a moving object through the
slits. Electronic stroboscopes consist of a light source whose flash
rate is controlled electronically. The
activities that follow can be used as individual student labs or as a
large class demonstration.
Get details on
Strobe Physics Tools here:
A stroboscope is able to “freeze” repetitive motions because it only
permits viewing at specific times. For example, if we are only allowed
to see an object each time it makes one complete rotation, the object
will always appear to be in the same place, and hence stationary. If the
viewing frequency is slightly greater than the object’s rotational
frequency, the object will appear to drift backward because it will be
seen before it is able to complete a complete rotation. Conversely, the
object will appear to drift forward if its frequency of rotation is
slightly greater than the viewing frequency. Most of us are familiar
with the apparent forward and backward motion of wheel covers on cars
when the imperceptible flashing of streetlights illuminates them.
To freeze motion with a mechanical
stroboscope, the rate of rotation of the strobe disk is adjusted until
the number of slits passing the eye of the viewer each second equals the
rate of the repetitive motion. For example, a fan will appear stopped if
the rate of viewing equals the rate of rotation of the fan.
When an electronic stroboscope illuminates a moving object in a darkened
room, the object will only be seen when the strobe light is on. When the
rate of flashing matches the rate of the repetitive motion, the object
will appear stopped.
If the viewing rate obtained with either type of strobe is known, it’s
possible to measure the short time required for one rotation of a fan
(or for one vibration of a tuning fork, or any other repetitive motion).
Here’s how to measure the frequency of a fan’s rotation with a hand-held
Put a distinguishing mark on one fan blade.
View the fan in motion through a rotating
Adjust the rate of rotation of the disc
until the marked blade appears stationary.
To insure that the rate of viewing is
synchronized with the motion of the object, a condition known as
resonance, increase the rate of rotation of the strobe disk until you
see two images of the blade. Reducing the rate of rotation of strobe
disk until a single image is seen will guarantee resonance. (Why?)
Have your partner use a stopwatch to
determine the time it takes for ten rotations
of the strobe disk.
Divide the number of rotations of the strobe
disk (10) by the time obtained in
step 5. This equals the number of rotations of the strobe disk per
Multiply the number of open slits in the
disk by the number of rotations per
second. This will yield the number of slits per second.
Because your viewing rate was synchronized
with the rate of the repetitive
motion, the number of slits per second equals the frequency of the fan’s
The period, or time required for one
complete rotation of the fan blade, is found by finding the reciprocal
of the frequency. For example, if the frequency equals 20 rotations per
second, the time required for one rotation is 1/20 second per rotation.
To measure the period of a repetitive motion,
in this case, a tuning fork, using an electronic stroboscope:
Strike the tuning fork and view it with the
Obtain resonance by adjusting the strobe’s
flash rate and read the flash rate from the strobe’s tachometer.
Find the reciprocal of the flash rate to
find the period of the motion.
Scientific online conversion calculators
The National Institute of Standards and
Technology ( http://www.nist.gov )
This site has a ton of interesting and useful information on units of
measure and other things that had to be standardized. (like the
color of traffic signals!).
Virtual Museum ( http://museum.nist.gov/ )
A Walk through Time:(
In the next issue of CoolStuff…
Each October physics teachers at New Trier and
Deerfield High Schools in Illinois and Creighton University in Omaha,
Nebraska, treat their students to a Haunted Physics Laboratory.
Visitors learn physics while having fun as they are confronted with a
maze of displays that demonstrate optical, acoustical, mechanical,
electrical, and perceptual phenomena in the context of Halloween.
Physics and non-physics students, their teachers, and the public become
engaged in trying to understand the science behind the fun.
In the next edition of CoolStuff we'll offer some examples of our favorite
Haunted Laboratory exhibits. The beauty of these displays is that they
are based on apparatus found in most science storerooms. With
slight modifications, many devices commonly used in the traditional
science laboratory may be transformed into something spooky. So
don't be afraid! Join us next time for a tour of our little lab of
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