Harmonic movement of a spring question

[transgalactic]

i got a mass "m" which is stuck to a spring which is hanged from the sealing .
k-the spring coefficient
the formula for acceleration is

a=-A*w^2*sin(wt+u)
i need to calculate the force which the spring exerts of the mass in any given moment
t=0 is when he was moved downward by distance A (amplitude) to some amplitude and released.

my main question is why the phase angle "u" is pi/4 ??

 

[Big-T]

Your initial conditions gives these equations, from which you should be able to retrieve u:
r(t)=Asin(wt+u)
r(0)=A

 

[transgalactic]

my main question is why the phase angle need to be pi/4 ??
how do i count the phrase angle in this string situation
just by viewing the location of the object.

 

[transgalactic]

m is mass
+ is the point of balance
what is the phase angle if we start counting from this position?
+
|
|
m

 

[Big-T]

Imagine the unit circle projected onto the y-axis, then pi/4 corresponds to 1, 0 and pi to 0 and -pi/4 to -1 as initial positions.

 

[transgalactic]

i can't image what you say
can you draw it??