Differential equation

[Logarythmic]

How do I solve

\ddot{\theta} = c \cos{\theta}?

[Integral]

Numerically. Try Runga Kutta, or for small angles Cos ( \theta ) ~ 1 so solve

\ddot{\theta} = c

[dextercioby]

How do I solve

\ddot{\theta} = c \cos{\theta}?

Multiply by 2\dot{\theta} and then notice that you get

\frac{d}{dt}\dot{\theta}^{2} = c\frac{d}{dt}{\sin\theta}

The rest is easy.

Daniel.

[Logarythmic]

Is it? =P

\dot{\theta}^2 = c_1 \sin{\theta} + c_2?

[dextercioby]

Yes, now separate variables and integrate.

Daniel.

[HallsofIvy]

"The rest is easy"!:rofl:

[Logarythmic]

Ok, so now I got the integral

\int \frac{d\theta}{\sqrt{c - \frac{3g}{L}sin{\theta}}}

to solve. Any tip?

[dextercioby]

Yes, use the web integrator from Mathematica's website:http://integrals.wolfram.com/index.jsp

The result is