# Frames of Reference

Before considering the difference between inertial and non-inertial frames of reference it is firstly important to consider what is a frame of reference and why is it important to understanding motion. A frame of reference is most easily defined as being that which other things are measured from. That means, for example, that we only know how fast we are going because we are able to compare it to those things around us. Consider the following, if a person is standing on the surface of the Earth next to a large tree, how fast are they moving? Most people would respond that they are not moving, that is because they have a velocity of zero relative to the tree and surrounds. However, they are actually moving relative to other frames of reference. For example they could be argued to be moving at nearly 460 m/s relative to the core of the Earth (this is the speed of the Earth’s rotation), or they could be said to be travelling at over 170,000 km/h relative to the Sun (that is the speed of the Earth’s orbit). So the original question is missing something about what frame of reference the question is considering.

The experience of relativity is often felt when you are on a train and your sole vision is of another train. You sometimes feel that you are moving, despite being stationary relative to the station, as the other train begins to leave. In such a situation there are two important components to considering what is happening. Firstly, who and where is the observer (O) and what frame or reference (R) are they referring to?

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