Addition: Equation for Pendulum Length

Would be cool to include this feature.

We can use this equation (referenced [here](https://phys.libretexts.org/Bookshelves/University_Physics/University_Physics_(OpenStax)/Book%3A_University_Physics_I_-_Mechanics_Sound_Oscillations_and_Waves_(OpenStax)/15%3A_Oscillations/15.05%3A_Pendulums))
```math
T = 2\pi \sqrt{\frac{L}{g}}
```

The issue identifies only the pendulum equation and an external reference, not a file, test, or entry point. First inspect the repository structure for existing teaching material or pendulum-related content, then determine where the equation belongs; done should mean the requested pendulum-length feature is included and its behavior or explanation is covered.

Written by the indexing model from the issue text.