Okay, so I have just started with simple harmonic motion(SHM). So the equation of displacement in my text book is given as: X= ACos(wt +x) where A is the amplitude X is displacememt from mean position at time t w is angular frequency x is phase constant. Everything going good till now, then comes an illustration where suddenly the equation becomes: X= ASin(wt +x) where A is the amplitude X is displacememt from mean position at time t w is angular frequency x is phase constant. and then onwards in most of the sums they solved with the sin equation and not cos. I dont get which equation to use when, how and why? I know every Sin function can be expressed in terms of cos and vice versa, also every SHM is either in the form of Sin or Cos. But I cant figure out which one to use and why and when. One observation i made- usually in case of linear SHM about X axis, cos equation is used and in case of linear SHM about y axis, sin equation is used. I used the word usually beacuse that was not the case everytime. Also i dont think this can be a logic as its upto me which one will i take as X axis and which one y if its just a linear SHM. Someone please help!!!!!!!