15. A travelling wave is described by the equation y(x, t)= 0.003 cos (20 x + 200t) where y and x are measured in meters and t in seconds. What is the period of this wave?

T = 3.14 10⁻²  s

Explanation:

The general equation for a traveling transverse wave is

           y = A sin (k x - wt +Ф)

where w is the angular velocity that is related to the period and Ф is the phase of the oscillation

           w = 2π / T          (1)

In this case they indicate that the expression of the wave

         y = 0.003 cos (20 x + 200t)

if we use the relationship of the double angles

          a = kx - wt

          b = Ф

          sin (a + b) = sin a cos b + sin b cos a

for the case Ф = 90 we have

          sin (a + b) = cos b

we substitute

          y = A cos (kx -wt)

with an initial phase of fi = 90º

if we compare the terms of the two expressions

         A = 0.003 m  

         k = 20 m⁻¹

         w = -200  rad/s

the negative sign indicates that the wave goes to the left

if we clear from equation 1

          T = 2π / w

          T = 2π / 200

          T = 3.14 10⁻²  s

the answer is a. at the equator