|
|
S E C T I O N 16 . 2 • Sinusoidal Waves 493 By definition, the wave travels a distance of one wavelength in one period T. There- fore, the wave speed, wavelength, and period are related by the expression (16.6) Substituting this expression for v into Equation 16.5, we find that (16.7) This form of the wave function shows the periodic nature of y. (We will often use y We can express the wave function in a convenient form by defining two other quan- tities, the angular wave number k (usually called simply the wave number) and the angular frequency ': (16.8) (16.9) Using these definitions, we see that Equation 16.7 can be written in the more compact (16.10) Using Equations 16.3, 16.8, and 16.9, we can express the wave speed v originally given in Equation 16.6 in the alternative forms (16.11) (16.12) The wave function given by Equation 16.10 assumes that the vertical position y of an element of the medium is zero at x ! 0 and t ! 0. This need not be the case. If it is y ! A sin(kx " 't # () (16.13) where ( is the phase constant, just as we learned in our study of periodic motion in Chapter 15. This constant can be determined from the initial conditions. v ! %f v ! ' k y ! A sin(kx " 't) '
% 2& T k % 2& % y ! A sin
! 2&
# x % " t T $ " v ! % T Angular wave number Speed of a sinusoidal wave General expression for a sinusoidal wave Wave function for a sinusoidal wave Angular frequency Quick Quiz 16.3 A sinusoidal wave of frequency f is traveling along a stretched string. The string is brought to rest, and a second traveling wave of frequency Quick Quiz 16.4 Consider the waves in Quick Quiz 16.3 again. The wave- length of the second wave is (a) twice that of the first wave (b) half that of the first wave |