Understanding Relative Motion
so it is almost like I am carrying a frame around me and I am seeing entire world with this single frame. And I keep this frame as a reference, so another way of think a point of view is that by calling them the frame of reference. And now if want to get more rigorous if call one of the edge as Y axis and one of the edge as X axis where I can measure things if I call that corner the origin then what I can do is to fix it somewhere and it become my frame of reference. if nothing is mentioned this frame is attached to earth, if it is assumed to attached to earth, which means anything that move with respect to earth I say it is moving. And anything which not moving the particular that I say, at rest. But if I take the frame of reference, and I attached to a car which is moving in some velocity then to arrive all other velocities with reference to that car, I must subtract the velocity of that car from the point of velocity of the earth to all the other velocities. That will give me the velocities in the frame of reference of the car. So mathematically if I say Va is velocity of a then assumed with velocity of earth, not mentioning anything else. And then if I have Vb as well as velocity of some other objects let’s see a car with respect to earth. Then that is also just going to be Vb. But if I want Vab, which is basically the velocity of a from the point of view of b. then what must I do? I will call that Vab and how do I calculate that? I have to subtract velocity of b from velocity of a because I am going to the point of view of b. that will become Va-Vb. So then I can call this velocity with respect to some frame as that object’s relative velocity. But every velocity is relative in some sense and in some frame of reference. But if this frame reference, which is a special one, if it is the earth then I drop the word relative because most of the measurements are from the point of view of earth.