OK, I'm working on a question regarding IRFs, but I seem to be a little confused. Question: Observer A is on the ground and Observer B in on a train moving with uniform velocity v wrt the ground. Each observes that a particle of mass m, initially at rest wrt the train, is acted upon by a constant force F applied to it in the forward direction for a time t. a)What is the work done on the particle by F in referece frame A and B? Are they equal? b)What are the changes in kinetic energy observed by A and B? c)Does the Work-energy theorem hold in reference frames of observers A and B? So, as I understand it, the particle is on the train as it is initially at rest wrt the train (i.e. it is moving with the train). The Force applied in the positive direction indicates that the train has slowed or stopped causing it to jerk forward. Here is where I get confused, how is the work different for each reference frame? If the train is moving in the +x direction, then the particle is also moving in the +x direction, then does A see the train move in the +x and B see the train move in -x direction? How does this change kinetic energy? Thanks!