Hello all, I just need some help understanding inertial frame of reference, is this when there is no acceleration acting on a body. such as, gravity.. but the body can be under constant velocity or at rest. Also, Could someone please help me with this senario: there is a guy in a rocket, bill, he takes off a constant velocity and then there is a girl, ellen, whom is watching the rocket. To bill, ellens watch seems to be going slow, but to ellen bills watch seems to being slow. what is the cause of this? Thanks alot.