Determining the Position as a Function of Time Equation

[jmason52]

In simple harmonic motion of a spring, the Position as a Function of Time Equation is: x(t) = Acos(wt+phi). How does one determine whether to use "cos" or "sin" when setting up the basic equation from the data given in a problem? Is it as simple as: use "sin" when x starts at equilibrium position (0), otherwise use "cos"? Can one of you offer me a simple clarification? Thanks!

 

[rock.freak667]

I believe it has to do with where it starts as you say. If it starts at the maximum point , you use cosine, if at 0 then sine.

 

[gneill]

You might consider using whichever makes the phase angle phi a nice value (magnitude, sign). Basically it's a matter of aesthetics unless some other part of the system under analysis forces your hand.