Damped Oscillation Equation: Finding Amplitude and Phase Angle

[Maxo]

Homework Statement

The equation for a damped oscillation is y(t)=Ae^{-\frac{b}{2m}t}cos(\omega&#039;t + \phi)<br />
We know that y(0)=0.5 and y'(0)=0.

Find the values of A and ø and then plot the oscillation in MATLAB.

Homework Equations

See above

The Attempt at a Solution

When derivating y(t) we get

y&#039;(t)=Ae^{-\frac{b}{2m}t}(-\frac{b}{2m}cos(\omega&#039;t+\phi)-\omega&#039;sin(\omega&#039;t + \phi))
This, together with the initial values gives that A=\frac{1}{2cos(\phi)} and \phi=arctan(-\frac{b}{2m\omega&#039;})

Is this correct? If not, then what is wrong?

 

[TSny]

This looks correct to me. As a check, you should see that your plot of y(t) satisfies the initial conditions.

 

[Maxo]

Ok, good. Then in other words, we get the following equation: y(t)=\frac{1}{2cos(arctan(-\frac{b}{2m\omega&#039;}))}e^{-\frac{b}{2m}t}cos(\omega&#039;t + arctan(-\frac{b}{2m\omega&#039;}))<br />
Right?

Now I wonder, how can we plot this in MATLAB? Can it be written just like it's written here? Because when I try I don't get a damped oscillation, but an undamped one. What is wrong then?

 

[TSny]

Ok, good. Then in other words, we get the following equation: y(t)=\frac{1}{2cos(arctan(-\frac{b}{2m\omega&#039;}))}e^{-\frac{b}{2m}t}cos(\omega&#039;t + arctan(-\frac{b}{2m\omega&#039;}))<br />
Right?

Yes.

Now I wonder, how can we plot this in MATLAB? Can it be written just like it's written here? Because when I try I don't get a damped oscillation, but an undamped one. What is wrong then?

I would need to see your code. I don't have MATLAB, but when I plot the function using other software, it looks OK.

 

[Maxo]

I don't have MATLAB, but when I plot the function using other software, it looks OK.

Could you please post your code for this? I will try some more and if I still don't manage I will post my code afterwards.