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Periodic Functions by: Doris Santarone

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To celebrate the new millennium, British Airways announced in 1996 its plans to fund construction of the world’s largest Ferris wheel. The wheel was designed to measure approximately 500 feet in diameter and to carry up to 800 passengers in 32 capsules. The wheel was designed to turn continuously and to be slow enough for people to hop on and off while it turns, completing a single rotation once every 20 minutes. Suppose you board this Ferris wheel at ground level. Let t represent the time since you boarded (in minutes) and let h represent your height above ground level (in feet). Ground Level Complete the table: t (min) 5 10 15 20 25 30 35 40 h (feet)

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**Generalize the behavior of the values of h. Is there a pattern**

Generalize the behavior of the values of h. Is there a pattern? Extend your table to include four of these repeated cycles. What would this look like if graphed?

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**Draw and label a scatter plot of h versus t.**

Observations?? What would be a good description of the shape of the graph if the data points were connected with a smooth curve? What natural phenomena have this shape? What characteristics will we be interested in?

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Since the Ferris wheel data is periodic, we can use a periodic function to model the relationship between h and t. Either the cosine function or the sine function will do, but we need to learn a little more about variations of these functions. We need some more terminology…

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Definitions: The period (or wavelength) of f is the length of one complete cycle. The midline is the horizontal line midway between the function’s minimum and maximum values. The amplitude is the distance between the function’s maximum (or minimum) value and the midline. The phase (or horizontal) shift is the number of units that the “start” of the cycle is away from being at the midline.

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We can use these values to modify the basic cosine or sine function in order to model our Ferris wheel situation.

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**For now, let’s simplify things and only consider the sine function**

For now, let’s simplify things and only consider the sine function. Similar relationships may be noticed for the cosine function. The applet linked below can help demonstrate how changes in these parameters affect the sinusoidal graph:

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**Let’s find an equation for the graph**

of a sinusoidal function. We'll do this one together. Let's do it step-by-step… Determine the amplitude _________ This is |a|. Determine the midline ___________ This is k. Determine the period ____________ Use this value to determine b. Locate a point on the graph that is at the midline and on an increasing interval. From this point we can determine the horizontal shift: ______ units left/right (which way?) Now we can determine h. Using the information above, you can determine a possible sinusoidal equation: Check by graphing your function on your calculator. Set an appropriate window based on the axis values that are shown above. Be sure that your calculator MODE is set to Radian!

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**Try to come up with an equation that models our Ferris wheel situation**

Try to come up with an equation that models our Ferris wheel situation. Check your equation on your graphing calculator. Questions: Compare your equation with another student or group. Is your solution unique? What is your predicted height above the ground when you are 12 minutes into the ride? Since the Ferris wheel rotates at a constant speed, when are you rising or falling the fastest? When are you rising or falling the slowest?

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Try to come up with a possible equation that will model h as a function of t. Check using your calculator. Compare with another group CLICK HERE for a GSP Sketch of Ferris Wheel.

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**Video: Teaching Mathematics 9-12 “Ferris Wheel”**

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Video Discussion Discuss the methods used by Ms. Compton to promote discourse and understanding. How does learning mathematics in context allow students to monitor their own thinking? Why is it important to question assumptions in problem solving situations?

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Many natural phenomena display periodic behavior similar to the up-and-down motion of a Ferris wheel seat. Some examples: Back-and-forth motion of a pendulum or swing Up-and-down motion of a weight bouncing on a spring Hours of daylight each day over the course of a year Depth of the tide at a particular location during the course of a day Flickering of a fluorescent light Vibration of a guitar or piano string

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