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Unformatted text preview: Physics 1240 Hall Chapter 2 Notes 1 Focus questions and learning goals Chapter 2 focus questions: 1. What is a wave? 2. How is a wave physically and mathematically described? Chapter 2 learning goals. After studying this chapter, you should be able to: 1. Look at a waveform on an oscilloscope screen and describe how you perceive the corresponding sound. (a) Determine the pitch. (b) Describe the loudness. (c) Determine whether the sound is a tone or noise. 2. Observe, describe, and predict properties a mass freely oscillating on the end of a spring. (a) Represent the motion in words and graphs. (b) Predict the tone you’ll hear if the mass is vibrating in air. (c) Predict how changing the stiffness of the spring or the weight of the object will change the tone. 3. Observe, describe, and predict properties a driven mass oscillating on the end of a spring. (a) Predict what happens as a result of the driving. (b) Explain how to make the motion very strong (and correspondingly, make the sound it produces loud). 2 Frequency and period In this section of the class we’ll focus on period and frequency of waves. These are properties of any periodic signal, which means that something repeats itself over and over with a well defined period. The signal can be smooth and simple (a perfect sine wave , like in Fig 2.12a) or a more complex but still repetitive signal (like in Fig 2.4 and 2.5). We already talked about these in the last chapter: The period is the time it takes for one full cycle. You can think of this as the time it takes to get back to where you started. We’ll often represent period with the symbol P . The frequency is the number of cycles that occur per second. We’ll often write frequency with the symbol f . Mathematically, f = 1 /P ; in words, frequency is the inverse of the period. Frequency is measured in units of Hz. 1 Hz = 1 cycle/sec. Humans can hear periodic pressure waves from roughly 20 Hz to 20 kHz. We’ll often use the kHz abbreviation. 1 kHz = kiloHz = 1,000 Hz = 10 3 Hz. The range of human hearing depends, of course, on your age, the loudness, and whether you’ve experience hearing loss, for example. 1 Physics 1240 Chapter 2 notes 2.1 Frequency, wavelength, and speed In studying chapter 2 we’ll discuss one of the most important relationships in any study of waves. This key relationship is the statement that wavelength times frequency = speed (for any wave). In symbols, we write this key relationship fλ = v. It’s important to understand why this formula is true. Please think about this carefully. Try to imagine a wave. To give us a good visual image, let’s imagine water waves. You can think of it in various ways. For example, you could zoom in on one point, where you will see the water bob up and down periodically with frequency f . Or, you can take a more holistic view, and watch the waves traveling along in front of you. If you focus on a specific peak or crest , you will see it move along in space, traveling outward. Alternatively, you could take a snapshot. Your snapshot doesalong in space, traveling outward....
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This note was uploaded on 08/24/2010 for the course PHYS 1240 taught by Professor Holland,murray during the Spring '08 term at Colorado.
- Spring '08