This tells us she has a velocity relative to the reference frame of the train. Let’s now say the person gets up out of her seat and walks toward the back of the train at 2 m/s. If we choose east as the positive direction and Earth as the reference frame, then we can write the velocity of the train with respect to the Earth as v → TE = 10 m / s i ^ v → TE = 10 m / s i ^ east, where the subscripts TE refer to train and Earth. Take the example of the person sitting in a train moving east. We introduce relative motion in one dimension first, because the velocity vectors simplify to having only two possible directions. We now develop a method to refer to reference frames in relative motion.
In summary, all discussion of relative motion must define the reference frames involved. In this case, the solar system is the reference frame. We can expand our view of the motion of the person on the train and say Earth is spinning in its orbit around the Sun, in which case the motion becomes more complicated.
If you say a person is sitting in a train moving at 10 m/s east, then you imply the person on the train is moving relative to the surface of Earth at this velocity, and Earth is the reference frame. In most examples we have examined so far, this reference frame has been Earth. When we say an object has a certain velocity, we must state it has a velocity with respect to a given reference frame. To discuss relative motion in one or more dimensions, we first introduce the concept of reference frame s. To explore this idea further, we first need to establish some terminology. Your velocity relative to the other train is 5 m/s west.
However, if another train passes you at 15 m/s east, your velocity relative to this other train is different from your velocity relative to the ground. If you’re riding in a train moving at 10 m/s east, this velocity is measured relative to the ground on which you’re traveling.
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