When to use sin vs cos in simple harmonic motion equations?

[Tricks67]

In what kind of cases do u we formulate the equation type x=Asin(wt+phi) or x=Acos(wt+phi)...and if for example we use it to define the position of some thing in motion when do we know when to use cos and when to use sin...?

 

[AlephZero]

It doesn't really matter which you use. The only difference is what position on the wave corresponds to phi = 0. A sin(wt+phi + pi/2) = A cos(wt+phi)

 

[jtbell]

when do we know when to use cos and when to use sin...?

It depends on the initial conditions for your specific situation. If the object is at x = 0 when t = 0, then it's easiest to use a sine wave. If the object is at x = maximum (i.e. the amplitude) when t = 0 then it's easiest to use a cosine wave.

However, as AlephZero noted, you can use either kind of wave in any case, by including an appropriate phase factor.

 

[Tricks67]

and we find the phase factor by setting x=0, right?