Motion is Relative

Frames of Reference

How fast is your chair moving? Compared to you, when you’re sitting in it, it had better be stationary, or you’ll end up on the floor pretty quickly! But, even if your chair is sitting still relative to the room and to the ground outside, the Earth is rotating at a great speed (almost 1700km/hr at the equator) and the Earth is orbiting the sun at about 28 km/second (that’s over one hundred thousand km/hr)!

And the sun is moving around the center of the galaxy!

And the galaxies are all moving too!

Velocity is only sensibly defined when it is relative to something – generically, this thing is referred to as a “reference frame”.

For example, in the frame of the room, your chair is stationary, but in the frame of the sun, it is moving at about 28km/second.