Defining Amplitude and Angle for Differential Equation x(t) = Asin(Wt + z)

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SUMMARY

The discussion focuses on defining the amplitude (A) and angle (z) in the differential equation x(t) = Asin(Wt + z) given initial conditions x(0) = x0 and x´(0) = v0. The user successfully derived the angle z using the equation z = tan^-1(x0W/v0) based on the simultaneous equations from the initial conditions. For amplitude A, the user rearranged the equations to A = x0/sin(z) and A = v0/Wcos(z), leading to the conclusion that A can be expressed in terms of x0, v0, and W by substituting the value of z.

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Dissonance in E
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Homework Statement


x(t) = Asin (Wt + z)
Define values for A(amplitude) and z(angle) so that:
x(0) = x0 and x´(0) = v0


Homework Equations


Chain rule.


The Attempt at a Solution


Im pretty sure I got the z part right:
x(0) = Asin(z) = x0
x´(0) = AWcos(z) = v0

Solve simultaneous equations for z:

sin(z)/cos(z) = (x0/a)(AW/v0)
tan(z) = (x0W/v0)
z = tan^-1(x0W/v0)

As for the A part I have no idea how to go about solving it:

Rearranging the x(0) equations for A
Asin(z) = x0
AWcos(z) = v0
A=x0/sin(z)
A=v0/Wcos(z)

Now what?
 
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Substitute the value you got for z into the equation A sin z = x0. That will give you A sin(tan-1(x0W/v0) = x0. Then solve that equation for A, which will give you A in terms of x0, v0, and W.
 

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