since the cosine function is simply the sine function with a phase difference of π/2. This is illustrated in Figure 5.7, which shows snapshots of Equations (5.14) and (5.16) at t=0. We simply need to choose the solution that his the initial conditions. Finally, in Section 3.6 we saw that it can be advantageous to use a complex representation of periodic motion. This is also the case for wave motion, remembering that. as usual, the real part of the complex form is the physical quantity. Thus we can write the following alternative mathematical expressions for travelling sinusoidal waves: