1) A wave expands as it moves farther from the source. Conserved energy is distributed over a larger area, decreasing intensity and amplitude of the wave. 2) Energy is lost due to friction and viscous forces as the wave travels.
Work Step by Step
1) See the discussion in Section 11–9 for spherical waves in 3-D. In 2-D, as the circular wave expands, the energy it carries is distributed over a larger and larger circumference, so the wave intensity has to decrease. Because the wave intensity is proportional to the square of the amplitude, the amplitude decreases also. 2) As the waves travel, the amplitude also decreases because the wave’s energy is dissipated through frictional energy loss.