Download Unit 4 Ch. 25 - Pendulum Activity

Lesson: Pendulum
Field Museum Extensions
a. Related Exhibitions.
1. Rocks and Minerals Exhibit—Earthquakes. Earthquakes occur
everywhere, even in Illinois. Some locations around the world experience
frequent and strong tremors; in other locations, earthquakes are rare, and
then, they are so small that most people don’t feel them. Even the smallest
earthquakes can be measured with a seismometer (also called a
seismograph). How a seismometer works is shown in this exhibit.
Students will carefully study the description of an earthquake and how a
seismometer works. Then, students can explain how the seismometer
shown is a pendulum and how it is similar and different from the
pendulum they have studied in the lab. Also, students can explain the
transfer of energy that occurs during an earthquake. (A pre-trip activity
involving the Shake, Rumble, and Roll Experience Box from Harris Loan,
along with the activity described below, is strongly recommended.)
The seismometer shown is a horizontal pendulum, different from the
vertical pendulum students have studied in lab. The horizontal pendulum
is often likened to a swinging fence gate, with the pendulum’s attachment
being the gate’s hinges. The seismometer’s “bob” (which would be the
hanging mass in a vertical pendulum) is stationary, while the support
(along with the recording paper) “swings” back and forth during an
earthquake. Thus, the bob is an inertial mass, attached to its support with
relatively frictionless connections.
Modern seismometers employ magnets and circuits to detect even the
smallest motion relative to the inertial mass. Levers are used to magnify
tiny tremors into the characteristic wavy pen recordings of a seismogram.
If students have completed the activity described with the Harris
Experience Box, they will note that the important parts of the pendulum
here are the length and the size of the inertial mass.
Earthquakes are the result of rock slabs below the surface being released
after great stress has been built up. The stress occurs when parts of the
earth’s crust are in motion relative to each other. A large rock slab in one
part of the crust gets stuck in another part. Since the elasticity of rocks is
so small, tension builds very quickly. Eventually, the rock is released and
vibrates.
In terms of energy transfer, the potential energy of the rock builds as the
crustal masses move, much like the potential energy of a rubber band
builds as it is being stretched. This potential energy is converted to kinetic
energy when the rock is released. The motion of the rock causes the crust
around it to move, and this can be felt at the surface as an earth tremor.
b. Harris Educational Loan Center.
1. Shake, Rumble, and Roll Experience Box. The contents of this box
include audiovisual materials and supplements on earthquakes, a fault
model, and a model of a seismometer. The seismometer model should be
investigated as a pendulum. The traditional vertical pendulum was also
used in the past to measure an earthquake. Students should describe the
difference between the seismometer pendulum and the traditional vertical
pendulum. They should explain the similarities and differences between
the laboratory pendulum and one that is being used as a seismometer.
Students should also set up their laboratory pendulum on a moveable ring
stand and use it as a model of a seismometer. Have students create a
hypothesis as to the important variables to consider for the pendulum: is
length important? Is bob mass important to correct function?
As students investigate the model seismometer, they should see it as being
similar to the pendulum model they have used in lab, with the difference
being that the seismometer is a pendulum turned on its side. This type of
pendulum is called a horizontal pendulum. Some necessary parts of a
horizontal pendulum include a rigid support for the “bob” (also known as
the inertial mass), additional support to keep the pendulum horizontal, and
a low friction point where the pendulum attaches to the frame.
This seismograph model is set into motion by rocking the base back and
forth, as if there were an earth tremor. Ask students to observe the
seismometer as you shake it with various vibrational speeds. Ask them if
the seismometer seems to work best at a certain speed. Ask them if there
is some variable in the construction of the seismometer that could be
changed for tremors of very high or low rates of movement. To test these
variables, have students experiment with their vertical pendulums.
Early versions of seismometers took on all forms, including vertical
pendulums. So the pendulum that they have investigated in lab would be a
model of an early seismometer. Have students create a small tremor by
moving the base of their pendulum back and forth, gently and with a
constant speed. Put the base on a notebook to avoid scratching the lab
table. Then, tell students to adjust anything that might make their
pendulum-turned-seismometer work better. A seismometer that functions
correctly has the inertial mass motionless while the stand “shakes”.
Students can try different bobs and different lengths. Eventually, they
should realize that the pendulum length is most important for a
seismometer to work correctly. (In a frictionless pendulum with small
swings, inertial mass size does not affect the rate of swing (or the period
of the pendulum). However, in a real pendulum, very light bobs are easily
set into motion by small frictional forces and string tensions. Therefore, a
more massive bob makes for a better seismometer.)
Eventually, students should notice the correlation between the rate of their
“tremors” and the period of the pendulum. They should notice that there
is a range of vibration rates that work best and that outside of this range,
the inertial mass begins to move, creating wild swings or a bob motion
that counters the motion of recording paper. In other words, the natural
period of the pendulum (which is related to its length) determines the best
seismometer. Once students discover this, have them find the best length
of their pendulum to match their tremor rate. Students may attach a pen to
their bob and pull a piece of paper past it while they shake the pendulum
to create a seismogram.
Of course, the equation for the period of their vertical seismometer is
different from the equation for a horizontal pendulum. Students should
remember that the period of a pendulum, T, is related to pendulum length,
L, and gravitational acceleration, g, by
L
T  2
g
In a horizontal pendulum, T does not depend on “g” because the
gravitational force is perpendicular to the movement. T depends on the
tension in the bob support and on the friction between the support and the

attachment point.
If “L” is small, the support is displaced from the inertial mass by a large
angle. Friction and support tension may cause motion in the inertial mass
as the support continues to move. The inertial mass and recording pen
would then swing off scale as well as move with the support, and a
readable seismogram would not be possible. (You may have noticed this
as you demonstrated the seismometer included in the Experience Box.) So
“L” should be large, but not so large that it bends under the weight of the
inertial mass. In constructing the seismometer, scientists must consider
the natural period of the seismometer to be within the range of ground
tremor rates.
The mass of the “bob” (the inertial mass) does not affect the period, but it
should be large enough so that the swinging support does not provide a
force (through friction) that can easily set it into motion.
2. Earthquakes Book, by S. Van Rose. A simple explanation of the cause,
locations, and consequences of earthquakes around the globe. Written for
elementary levels, but useful as a quick read.
c. Field Museum Science/Website Resources.
1. Nature Unleashed Webpage
(http://www.fieldmuseum.org/natureunleashed/earthquakes.asp) Find out
more about the devastating effects of earthquakes. Also, a short
description of seismometers and seismographs is included.
The educator guide can be downloaded at
http://www.fieldmuseum.org/education/guides/NU_educator_guide.pdf
2. National Geographic’s Forces of Nature Website:
http://environment.nationalgeographic.com/environment/naturaldisasters/forces-of-nature.html?section=e
Learn more about the origin of earthquakes, seismic waves, and
seismometers and seismographs. This site has an interactive graphic that
demonstrates the causes, locations, and effects of ground tremors.
Students can trigger their own quake and see the results.
3. Dynamic Earth: the Story of Plate Tectonics, The United States
Geological Survey website:
http://pubs.usgs.gov/gip/dynamic/dynamic.html
More information on what makes the earth move, as well as the results of
plate movement. Students may use the information from this site to help
explain the transfer of energy occurring below the surface of the earth.
This is a good companion to the Field Museum’s exhibit The Moving
Earth, and the Harris Loan movie of the same name. This site also has
many graphics that teachers may want to project in class for background
information as they explain the seismometer.