remove unused equation file

\documentclass[preview]{standalone}

\usepackage{varwidth}

\usepackage[utf8x]{inputenc}

\usepackage{amsmath, amssymb, graphics, setspace}

\begin{document}

\begin{varwidth}{50in}

  \begin{equation}

    \frac{d \vec{s}_{i}}{dt} = \frac{1}{\left(1+\lambda^2 \right)} \left( \left(

    \vec{\omega}_{i} +\vec{\eta} \right) \times \vec{s}_{i} + \lambda\, \vec{s}_{i}

  \times\left( \vec{\omega}_{i} \times\vec{s}_{i} \right) \right), \nonumber

  \end{equation}

\end{varwidth}

\end{document}

\documentclass[12pt]{article}

\usepackage{amsmath}

\begin{document}

$$

  F = \left( 1-\lambda \right) F_{\text{solid}} + \lambda F_{\text{harm}}

$$

\end{document}