In lecture, we are beginning to learn about waves and periodic motion under simple harmonic motion. We were given the equations: x=Acosθ and θ=ωt+\phi -- Substituting, we get x=Acos(ωt+\phi). This is simple enough; however what is Phi? All I was told is that "phi is a constant that allows us to start anywhere on the sine or cosine graph," meaning that the phi term allows the equation to work with both sine and cosine functions. This being said, what exactly is phi? In using this equation to solve problems, how do I know what to set for phi if it seems to be just a phase shift to compensate for the sine and cosine functions? Is it something that I can arbitrarily choose to be 0 if I am not given a specific value to use in the question? Thank you in advance for your responses.