# Can someone explain to me what my professor did?

1. Nov 6, 2015

### Warlic

1. The problem statement, all variables and given/known data

Two things I dont understand; how did he get that omega is sqrt(k/m-(b/2m)^2)
And second; why is it that x(t) = e^(-bt/2m)* cos (omega*t+phi)
shouldnt it rather be; x(t) = c1*cos(ωt) + c2*sin(ωt)

2. Relevant equations

3. The attempt at a solution

2. Nov 6, 2015

### Warlic

I figured out why omega is what it is, but still dont understand how he got the equation for x(t)

3. Nov 6, 2015

### Staff: Mentor

I think he's assuming that the system is underdamped, so will be a damped sinusoid. The general solution can be molded into that shape:

$x(t) = c_1 cos(ωt) + c_2 sin(ωt)$
$~~~= \sqrt{c_1^2 + c_2^2}\left( \frac{c_1}{\sqrt{c_1^2 + c_2^2}} cos(ωt) + \frac{c_2}{\sqrt{c_1^2 + c_2^2}} sin(ωt) \right)$
$~~~= \sqrt{c_1^2 + c_2^2}\left(cos(\phi) cos(ωt) + sin(\phi) sin(ωt) \right)$
$~~~= \sqrt{c_1^2 + c_2^2} cos(ωt - \phi)$ where: $~~~~\phi = tan^{-1}\left( \frac{c_2}{c_1} \right)$

You can fudge the sign of the angle $\phi$ by negating it and interpreting it appropriately.

4. Nov 6, 2015

### Mister T

Aren't c1 and c2 constants? If so, I see a sinusoid, not a damped sinusoid.

5. Nov 6, 2015

### Staff: Mentor

Ah, right. I left out the damping term in the general solution So:
$x(t) = e^{-\alpha t}(c_1 cos(ωt) + c_2 sin(ωt))$
Go from there. The roots of the auxiliary equation will be complex conjugates of the form $\alpha ± \omega$, where $\omega$ can be further broken down as $\omega = \sqrt{\alpha^2 - \omega_o^2}$

6. Nov 6, 2015

### Warlic

This is exactly the point where I dont know where to go from :P. Where does the c2sin(ωt) part go?

7. Nov 6, 2015

### Staff: Mentor

See post #3. The sin and cos terms can be amalgamated into a single cos (or sin) term with a constant phase shift.

8. Nov 6, 2015

### Mister T

Look at the undamped case:

$x(t)=c_1 \cos (\omega t)+c_2 \sin (\omega t)$

and

$x(t)=A \cos (\omega t+ \phi)$

are equivalent. Note that each contains two constants of integration.