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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.