# Derivation of expression for strain in a beam that experiences transverse vibrations

I was watching this video (the link to the video is https://www.youtube.com/watch?v=AoxTyIJprg8&list=PLMXj6GKKnHI6Lftj7CXr9WusMkXi5s9yH&index=60) which is part of a course on mechanical vibrations. Can someone explain how the expression for strain is derived in the video? Why is strain the derivative of u(x,t)?

• Are you interested why $\epsilon$ is a derivative of u, t, or are you interested about the derivation of bending strain, or both? – NMech Apr 5 at 5:49