# Problem with the Foucault Pendulum motion derivation

A bad drawing just so you can see what I mean:

On every derivation I see, at some point it is stated that: $$\frac{T_x}{T}=\frac{-x}{L}$$ It is always explained by saying "by the geometry of the problem" or "by triangle similarity" but I fail to see how that follows so easily. This relation implies that as the x component of the mass gets larger, more tension is in that axis. That of course makes sense, but how is it possible to explicitly see this geometrically?

It actually is straightforward geometry if you draw the figure properly. Look at $$\theta$$, the angle made by the pendulum with the $$zy$$ plane. It’s cosine is the required ratio.
• $\hat{x}$ and $\vec T_x$ point along opposite directions. Aug 20, 2020 at 21:12