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driving force is variable while the natural frequency ' 0 of the oscillator is fixed by the values of k and m. Newton’s second law in this situation gives (15.34) Again, the solution of this equation is rather lengthy and will not be presented. After the (15.35) where (15.36) and where is the natural frequency of the undamped oscillator (b ! 0). Equations 15.35 and 15.36 show that the forced oscillator vibrates at the frequency of the driving force and that the amplitude of the oscillator is constant for a given $ ' 0 . The dramatic increase in amplitude near the natural frequency is called resonance, and the natural frequency ' 0 is also called the resonance frequency of the system. The reason for large-amplitude oscillations at the resonance frequency is that en- ergy is being transferred to the system under the most favorable conditions. We can t % (), which is the same trigonometric function as that describing the driving force. Thus, the applied force F is in phase with the velocity. The rate at which work is done on the oscillator by F equals the dot product F # v ; this rate is the power delivered to the oscillator. Because the product F # v is a maximum when F and v are in phase, we conclude that at resonance the applied force is in phase with the velocity and the power transferred to the oscillator is a maximum. Figure 15.25 is a graph of amplitude as a function of frequency for a forced oscillator with and without damping. Note that the amplitude increases with decreasing damping 0 . In other words, if there are no losses in the system and if we continue to drive an initially Later in this book we shall see that resonance appears in other areas of physics. For ex- ample, certain electric circuits have natural frequencies. A bridge has natural frequencies ' 0 ! √ k/m A ! F 0 /m √ (' 2 " ' 0
2 ) 2 % " b' m # 2 x ! A cos('t % () ! F ! ma 9: F 0 sin 't " b
dx dt " kx ! m
d 2 x dt 2 S E C T I O N 15 . 7 • Forced Oscillations 473 Amplitude of a driven oscillator A b = 0 Undamped Small b Large b ω 0 0 ω ω Figure 15.25 Graph of amplitude versus frequency for a damped oscillator when a periodic driving force is present. When the frequency ' of the driving force equals the natural frequency ' 0 of the oscillator, resonance occurs. Note that the shape of the resonance curve depends on the size of the damping coefficient b. |