Energy, work and power — practice question

A device used to drive a nail into a block of wood makes a hammerhead fall straight down onto the head of a nail. The hammerhead has mass $1.2\,\text{kg}$ and the nail has mass $0.004\,\text{kg}$ (see diagram). The hammerhead strikes the nail with speed $v\,\text{m s}^{-1}$ and stays in contact with the nail after the collision. Immediately after the impact, the hammerhead and nail together move with speed $40\,\text{m s}^{-1}$.