1.4.A.1Concept
The choice of reference frame will determine the direction and magnitude of quantities measured by an observer in that reference frame.
ExampleA ball resting on a moving train1 / 3
A ball sits motionless on the floor of a train car moving at 20 m/s relative to the ground. What velocity does a passenger sitting next to the ball measure for it? What velocity does an observer standing on the platform measure for it?
Try it on paper first, then reveal each step to check your work.
Step 1 — The passenger's frame
In the train's reference frame, the ball isn't going anywhere relative to the passenger — the passenger measures the ball's velocity as 0 m/s.