Halliday, Resnick & Walker — Kinetic Energy and Work: Problem 77

Work, train observer, ground observer. An object with mass \( m \) is initially stationary inside a train that moves at constant speed \( u \) along an \( x \) axis. A constant force then gives the object an acceleration \( a \) in the forward direction for time \( t \). In terms of these given symbols, how much work is done by the force as measured by (a) an observer stationary inside the train and (b) an observer stationary alongside the track?