|
|
S E C T I O N 16 . 5 • Rate of Energy Transfer by Sinusoidal Waves on Strings 501 According to Equation 16.18, the speed of a wave on a string increases as the mass per unit length of the string decreases. In other words, a wave travels more slowly on a when a wave or pulse travels from medium A to medium B and v A ! v B (that is, when B is denser than A), it is inverted upon reflection. When a wave or pulse travels from medium A to medium B and A " v B (that is, when A is denser than B), it is not inverted upon reflection. 16.5 Rate of Energy Transfer by Sinusoidal Waves on Strings Waves transport energy when they propagate through a medium. We can easily demon- Consider a sinusoidal wave traveling on a string (Fig. 16.19). The source of the en- ergy is some external agent at the left end of the string, which does work in producing A. The kinetic energy K associated with a moving particle is . If we apply this equation to an element of length , x and mass ,m, we see that the kinetic energy ,K of this element is where v y is the transverse speed of the element. If + is the mass per unit length of the string, then the mass ,m of the element of length ,x is equal to + ,x. Hence, we can (16.19) As the length of the element of the string shrinks to zero, this becomes a differential We substitute for the general transverse speed of a simple harmonic oscillator using ! 1 2 +' 2 A 2 cos 2 (kx " 't) dx dK ! 1 2 + ['A cos(kx " 't)] 2 dx dK ! 1 2 (+
dx)v y 2 , K ! 1 2 (+ ,x)v y 2 , K ! 1 2 (,m)v y 2 K ! 1 2 mv 2 m m (a) (b) Figure 16.18 (a) A pulse traveling to the right on a stretched string that has an object suspended from it. (b) Energy is transmitted to the suspended object when the pulse arrives. ∆m Figure 16.19 A sinusoidal wave trav- eling along the x axis on a stretched string. Every element moves verti- cally, and every element has the same total energy. |